SbsaQemuRme: Add support for SbsaQemuRme platform

Add support for SbsaQemuRme platform by 1) pushing the start of the base
memory to 0x10043000000 to make room to the RMM and 2) increasing the
size of FLASH0 to accomotate the RMM.

Enabling RMM is done by specifying -D ENABLE_RME on the build command
line.  The name of the flag was chosen to reflect what is currently done
on TF-A.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>