dt-bindings: rtc: new binding for Amlogic VRTC

* Amlogic Virtual RTC (VRTC)

This is a Linux interface to an RTC managed by firmware, hence it's

virtual from a Linux perspective.  The interface is 1 register where

an alarm time (in seconds) is to be written.

Required properties:

- compatible: should be "amlogic,meson-vrtc"

- reg: physical address for the alarm register

The alarm register is a simple scratch register shared between the

application processors (AP) and the secure co-processor (SCP.)  When

the AP suspends, the SCP will use the value of this register to

program an always-on timer before going sleep. When the timer expires,

the SCP will wake up and will then wake the AP.

Example:

	vrtc: rtc@0a8 {

		compatible = "amlogic,meson-vrtc";

		reg = <0x0 0x000a8 0x0 0x4>;

	};