watchdog: bcm7038: add device tree binding documentation

BCM7038 Watchdog timer

Required properties:

- compatible : should be "brcm,bcm7038-wdt"

- reg : Specifies base physical address and size of the registers.

Optional properties:

- clocks: The clock running the watchdog. If no clock is found the

	  driver will default to 27000000 Hz.

Example:

watchdog@f040a7e8 {

	compatible = "brcm,bcm7038-wdt";

	clocks = <&upg_fixed>;

	reg = <0xf040a7e8 0x16>;

};