KVM: arm64: Reuse sys_reg() macro when searching the trap table

Instead of having an open-coded macro, reuse the sys_reg() macro
that does the exact same thing (the encoding is slightly different,
but the ordering property is the same).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>