The default Raspberry Pi 3 memory mapping for ATF is geared towards
the use of uboot + Linux. This creates issues when trying to use
ATF with an UEFI payload and Windows on ARM64.

We therefore introduce new build option RPI3_USE_UEFI_MAP, that
enables the build process to use an alternate memory mapping that
is compatible with UEFI + Windows (as well as UEFI + Linux).

Fixes ARM-software/tf-issues#649

Signed-off-by: Pete Batard <pete@akeo.ie>