ARM: meson: documentation: Add timer documentation

Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NCarlo Caione <carlo@caione.org>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/amlogic,meson6-timer.txt

+Amlogic Meson6 SoCs Timer Controller
+
+Required properties:
+
+- compatible : should be "amlogic,meson6-timer"
+- reg : Specifies base physical address and size of the registers.
+- interrupts : The interrupt of the first timer
+
+Example:
+
+timer@c1109940 {
+	compatible = "amlogic,meson6-timer";
+	reg = <0xc1109940 0x14>;
+	interrupts = <0 10 1>;
+};