rpi3: Move shared rpi3 files into common directory

To be able to share code more easily between the existing Raspberry Pi 3
and the upcoming Raspberry Pi 4 platform, move some code which is not
board specific into a "common" directory.

Change-Id: I9211ab2d754b040128fac13c2f0a30a5cc8c7f2c
Signed-off-by: Andre Przywara <andre.przywara@arm.com>