selftests: arm64: Add build and documentation for FP tests

Integrate the FP tests with the build system and add some documentation
for the ones run outside the kselftest infrastructure.  The content in
the README was largely written by Dave Martin with edits by me.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200819114837.51466-7-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>