kselftest/arm64: Add hwcap test for RNG

Validate the RNG hwcap and make sure we don't generate a SIGILL reading
RNDR when it is reported.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220913141101.151400-4-broonie@kernel.org


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