dt-bindings: Add binding for cros-ec-rpmsg

Add a DT binding documentation for ChromeOS EC driver over rpmsg.

Signed-off-by: Pi-Hsun Shih <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/cros-ec.txt

···
 Google's ChromeOS EC is a Cortex-M device which talks to the AP and
 implements various function such as keyboard and battery charging.
 
-The EC can be connect through various means (I2C, SPI, LPC) and the
+The EC can be connect through various means (I2C, SPI, LPC, RPMSG) and the
 compatible string used depends on the interface. Each connection method has
 its own driver which connects to the top level interface-agnostic EC driver.
 Other Linux driver (such as cros-ec-keyb for the matrix keyboard) connect to
···
 Required properties (SPI):
 - compatible: "google,cros-ec-spi"
 - reg: SPI chip select
+
+Required properties (RPMSG):
+- compatible: "google,cros-ec-rpmsg"
 
 Optional properties (SPI):
 - google,cros-ec-spi-pre-delay: Some implementations of the EC need a little