dt/bindings: Add binding for the Raspberry Pi firmware driver

This driver will provide support for calls into the firmware that will
be used by other drivers like cpufreq and vc4.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>