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>

To allow sharing the driver between the RPi3 and RPi4, move the random
number generator driver into the generic driver directory.

Change-Id: Iae94d7cb22c6bce3af9bff709d76d4caf87b14d1
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

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>

With the incoming support for the Raspberry Pi 4 boards, one directory
to serve both versions will not end up well.

Create an additional layer by inserting a "rpi" directory betweeen /plat
and rpi3, so that we can more easily share or separate files between the
two later.

Change-Id: I75adbb054fe7902f34db0fd5e579a55612dd8a5f
Signed-off-by: Andre Przywara <andre.przywara@arm.com>