dt-bindings: rtc: add device-tree bindings for the Amlogic Meson RTC

* Amlogic Meson6, Meson8, Meson8b and Meson8m2 RTC

Required properties:

- compatible: should be one of the following describing the hardware:

	* "amlogic,meson6-rtc"

	* "amlogic,meson8-rtc"

	* "amlogic,meson8b-rtc"

	* "amlogic,meson8m2-rtc"

- reg: physical register space for the controller's memory mapped registers.

- interrupts: the interrupt line of the RTC block.

- clocks: reference to the external 32.768kHz crystal oscillator.

- vdd-supply: reference to the power supply of the RTC block.

- resets: reset controller reference to allow reset of the controller

Optional properties for the battery-backed non-volatile memory:

- #address-cells: should be 1 to address the battery-backed non-volatile memory

- #size-cells: should be 1 to reference the battery-backed non-volatile memory

Optional child nodes:

- see ../nvmem/nvmem.txt

Example:

	rtc: rtc@740 {

		compatible = "amlogic,meson6-rtc";

		reg = <0x740 0x14>;

		interrupts = <GIC_SPI 72 IRQ_TYPE_EDGE_RISING>;

		clocks = <&rtc32k_xtal>;

		vdd-supply = <&rtc_vdd>;

		resets = <&reset RESET_RTC>;

		#address-cells = <1>;

		#size-cells = <1>;

	};