fix(docs): make required compiler version == rather than >=

TF-A carries its own compiler-rt so higher versions of the compilers may
not necessarily work. Because TF-A is only tested on the specified
versions in the CI, any breakage remains unknown. Update the
prerequisites guide to make it more apparent that this is the case.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ia5da9c5ff505ead99f579f3f5fbe3a480d697c1d