Modify FVP makefile for cores that support both AArch64/32

Modify FVP makefile for cores that support both AArch64/32

Some cores support only AArch64 from EL1 and above, e.g. A76, N1 etc. If
TF-A is compiled with CTX_INCLUDE_AARCH32_REGS=0 so as to properly
handle those cores, only the AArch64 cores' assembly is included in the
TF-A binary. In other words, for FVP, TF-A assumes that AArch64 only
cores will never exist in the same cluster with cores that also support
AArch32.

However, A55 and A75 can be used as AArch64 only cores, despite
supporting AArch32, too. This patch enables A55 and A75 to exist in
clusters together with AArch64 cores.

Change-Id: I58750ad6c3d76ce77eb354784c2a42f2c179031d
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>