rpi3: Add mem reserve region to DTB if present

rpi3: Add mem reserve region to DTB if present

When a device tree blob is present at a known address, instead of, for
example, relying on the user modifying the Linux command line to warn
about the memory reserved for the Trusted Firmware, pass it on the DTB.

The current code deletes the memory reserved for the default bootstrap
of the Raspberry Pi and adds the region used by the Trusted Firmware.

This system replaces the previous one consisting on adding
``memmap=16M$256M`` to the Linux command line. It's also meant to be
used by U-Boot and any other bootloader that understands DTB files.

Change-Id: I13ee528475fb043d6e8d9e9f24228e37ac3ac436
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>