selftests/arm64: Define top_srcdir for the fp tests

Some of the rules in lib.mk use a top_srcdir variable to figure out where
the top of the kselftest tree is, provide it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220427181954.357975-3-broonie@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>