ARM: bcm2835: Add the Raspberry Pi power domain driver to the DT.

This connects the USB driver to the USB power domain, so that USB can
actually be turned on at boot if the bootloader didn't do it for us.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kevin Hilman <khilman@linaro.org>