rpi3: Make SHARED_RAM optional
rpi3: Make SHARED_RAM optional

The existing Raspberry Pi 3 port sports a number of memory regions,
which are used for several purposes. The upcoming RPi4 port will not use
all of those, so make the SHARED_RAM region optional, by only mapping it
if it has actually been defined. This helps to get a cleaner RPi4 port.

Change-Id: Id69677b7fb6ed48d9f238854b610896785db8cab
Signed-off-by: Andre Przywara <94eb15b8f54b8e37c3f8d338296201be47dc5c3e@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware