arm64: Move ICH_ sysreg bits from arm-gic-v3.h to sysreg.h

These definitions are in arm-gic-v3.h for historical reasons which no
longer apply. Move them to sysreg.h so the AIC driver can use them, as
it needs to peek into vGIC registers to deal with the GIC maintentance
interrupt.

Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>