dt-bindings: rtc: new binding for Amlogic VRTC

Add binding fo the new VRTC driver for Amlogic SoCs.  The 64-bit
family of SoCs only has an RTC managed by firmware, and this VRTC
driver provides the simple, one-register firmware interface.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20190812232850.8016-2-khilman@kernel.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

+* 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>;
+	};