arm64/sysreg: Add defintion for ALLINT

The FEAT_NMI extension adds a new system register ALLINT for controlling
NMI related interrupt masking, add a definition of this register as per
DDI0487H.a.

Reviewed-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220905225425.1871461-29-broonie@kernel.org