KVM: arm64: Refactor sys_regs.h,c for nVHE reuse

Refactor sys_regs.h and sys_regs.c to make it easier to reuse
common code. It will be used in nVHE in a later patch.

Note that the refactored code uses __inline_bsearch for find_reg
instead of bsearch to avoid copying the bsearch code for nVHE.

No functional change intended.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210817081134.2918285-6-tabba@google.com