pinctrl: meson: add device tree bindings documentation

Add device tree bindings documentation for Amlogic Meson pin and GPIO
controller.
Signed-off-by: NBeniamino Galvani <b.galvani@gmail.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/meson,pinctrl.txt

+== Amlogic Meson pinmux controller ==
+
+Required properties for the root node:
+ - compatible: "amlogic,meson8-pinctrl"
+ - reg: address and size of registers controlling irq functionality
+
+=== GPIO sub-nodes ===
+
+The 2 power domains of the controller (regular and always-on) are
+represented as sub-nodes and each of them acts as a GPIO controller.
+
+Required properties for sub-nodes are:
+ - reg: should contain address and size for mux, pull-enable, pull and
+   gpio register sets
+ - reg-names: an array of strings describing the "reg" entries. Must
+   contain "mux", "pull" and "gpio". "pull-enable" is optional and
+   when it is missing the "pull" registers are used instead
+ - gpio-controller: identifies the node as a gpio controller
+ - #gpio-cells: must be 2
+
+Valid sub-node names are:
+ - "banks" for the regular domain
+ - "ao-bank" for the always-on domain
+
+=== Other sub-nodes ===
+
+Child nodes without the "gpio-controller" represent some desired
+configuration for a pin or a group. Those nodes can be pinmux nodes or
+configuration nodes.
+
+Required properties for pinmux nodes are:
+ - groups: a list of pinmux groups. The list of all available groups
+   depends on the SoC and can be found in driver sources.
+ - function: the name of a function to activate for the specified set
+   of groups. The list of all available functions depends on the SoC
+   and can be found in driver sources.
+
+Required properties for configuration nodes:
+ - pins: a list of pin names
+
+Configuration nodes support the generic properties "bias-disable",
+"bias-pull-up" and "bias-pull-down", described in file
+pinctrl-bindings.txt
+
+=== Example ===
+
+	pinctrl: pinctrl@c1109880 {
+		compatible = "amlogic,meson8-pinctrl";
+		reg = <0xc1109880 0x10>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges;
+
+		gpio: banks@c11080b0 {
+			reg = <0xc11080b0 0x28>,
+			      <0xc11080e8 0x18>,
+			      <0xc1108120 0x18>,
+			      <0xc1108030 0x30>;
+			reg-names = "mux", "pull", "pull-enable", "gpio";
+			gpio-controller;
+			#gpio-cells = <2>;
+               };
+
+		gpio_ao: ao-bank@c1108030 {
+			reg = <0xc8100014 0x4>,
+			      <0xc810002c 0x4>,
+			      <0xc8100024 0x8>;
+			reg-names = "mux", "pull", "gpio";
+			gpio-controller;
+			#gpio-cells = <2>;
+		};
+
+		nand {
+			mux {
+				groups = "nand_io", "nand_io_ce0", "nand_io_ce1",
+					 "nand_io_rb0", "nand_ale", "nand_cle",
+					 "nand_wen_clk", "nand_ren_clk", "nand_dqs",
+					 "nand_ce2", "nand_ce3";
+				function = "nand";
+			};
+		};
+
+		uart_ao_a {
+			mux {
+				groups = "uart_tx_ao_a", "uart_rx_ao_a",
+					 "uart_cts_ao_a", "uart_rts_ao_a";
+				function = "uart_ao";
+			};
+
+			conf {
+				pins = "GPIOAO_0", "GPIOAO_1",
+				       "GPIOAO_2", "GPIOAO_3";
+				bias-disable;
+			};
+		};
+	};