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

The 32-bit Amlogic Meson SoCs (Meson6, Meson8, Meson8b and Meson8m2)
have a built-in RTC block.
It has the following inputs:
- an 32.768kHz crystal oscillator
- an interrupt line
- a reset line
- 0.9V voltage input

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[resurrected patches from Ben after 2 years]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/rtc-meson.txt

@@ -0,0 +1,35 @@
+* 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>;
+	};