Minor clean-ups for ARM64 GICv3 and GIC drivers
GICv3:
- move ICC_SGI1R_EL1 definitions to armreg.h and use proper system register's names
GIC:
- remove unused functions
Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5119