rpi3: Move VC mailbox driver into generic drivers directory
Andre Przywara authored
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: default avatarAndre Przywara <andre.przywara@arm.com>
c0031189