arm64/sysreg: Support generation of RAZ fields

Add a statement for RAZ bitfields to the automatic register generation
script. Nothing is emitted to the header for these fields.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220510161208.631259-7-broonie@kernel.org


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