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 <robh+dt@kernel.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>