dt-bindings: mfd: Add bindings for ZII RAVE devices

Add Device Tree bindings for RAVE Supervisory Processor, an MCU
implementing varoius bits of housekeeping functionality (watchdoging,
backlight control, LED control, etc) on RAVE family of products by
Zodiac Inflight Innovations.

Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Andrey Smirnov <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/zii,rave-sp.txt

···
+Zodiac Inflight Innovations RAVE Supervisory Processor
+
+RAVE Supervisory Processor communicates with SoC over UART. It is
+expected that its Device Tree node is specified as a child of a node
+corresponding to UART controller used for communication.
+
+Required parent device properties:
+
+ - compatible: Should be one of:
+	- "zii,rave-sp-niu"
+	- "zii,rave-sp-mezz"
+	- "zii,rave-sp-esb"
+	- "zii,rave-sp-rdu1"
+	- "zii,rave-sp-rdu2"
+
+ - current-speed: Should be set to baud rate SP device is using
+
+RAVE SP consists of the following sub-devices:
+
+Device				 Description
+------				 -----------
+rave-sp-wdt			: Watchdog
+rave-sp-nvmem			: Interface to onborad EEPROM
+rave-sp-backlight		: Display backlight
+rave-sp-hwmon			: Interface to onboard hardware sensors
+rave-sp-leds			: Interface to onboard LEDs
+rave-sp-input			: Interface to onboard power button
+
+Example of usage:
+
+	rdu {
+		compatible = "zii,rave-sp-rdu2";
+		current-speed = <1000000>;
+
+		watchdog {
+			compatible = "zii,rave-sp-watchdog";
+		};
+	};
+