arm64/sysreg: Directly include bitfield.h

The SYS_FIELD_ macros in sysreg.h use definitions from bitfield.h but there
is no direct inclusion of it, add one to ensure that sysreg.h is directly
usable.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220818213613.733091-2-broonie@kernel.org


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