dt-bindings: add rpi power domain driver bindings

This patch adds devicetree tree bindings for the Raspberry Pi power
domain driver.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Kevin Hilman <khilman@linaro.org>