feat(hcx): test that HCRX_EL2 is accessible

On systems with FEAT_HCX support this CPU extension test attempts
to access HCRX_EL2, if it can then the test passes, if not the
system traps to EL3 and crashes. This tells us whether or not TF-A
has properly enabled access to the register. To enable this test,
compile TFTF with TESTS=hcx, this is done to prevent this test from
running automatically on systems which may support FEAT_HCX but do
not have it enabled in TFA.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: If5787c3e947872169af3c9c3e57587cbcd41fd9e
6 files changed