mfd: syscon: Add Atmel GPBR DT bindings documention

The GPBR block provides a set of battery-backed registers that can be used
to save data which need to be kept when the system is powered down and
VDD-core is maintained by an external battery.

A typical usage is the RTT block (when used as an RTC) which needs one of
those registers to save the current time.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: NJohan Hovold <johan@kernel.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/atmel,gpbr.txt

+* Device tree bindings for Atmel GPBR (General Purpose Backup Registers)
+
+The GPBR are a set of battery-backed registers.
+
+Required properties:
+- compatible:		"atmel,at91sam9260-gpbr", "syscon"
+- reg:			contains offset/length value of the GPBR memory
+			region.
+
+Example:
+
+gpbr: gpbr@fffffd50 {
+	compatible = "atmel,at91sam9260-gpbr", "syscon";
+	reg = <0xfffffd50 0x10>;
+};