rpi3: Move VC mailbox driver into generic drivers directory

To allow sharing the driver between the RPi3 and RPi4, move the mailbox
driver into the generic driver directory.

Change-Id: I463e49acf82b02bf004f3d56482b7791f3020bc0
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
5 files changed