fix(rme): relax RME compiler requirements

Currently building TF-A for the FVP with RME enabled requires a
toolchain that understands the -march=armv8.6-a command line option,
even though we actually don't need any ARMv8.6 features from the
compiler.

Relax the requirement to use ARMv8.5, since this is what's the GCC
shipped with Ubuntu 20.04 understands. This is in line what the current
RMM implementation uses as well.

Change-Id: I3806dcff90319a87f003fe2c86b7cdcdebd625e4
Signed-off-by: Andre Przywara <andre.przywara@arm.com>