KVM: arm64: move cp15 emulation of ACTLR{,2} to the cp15_regs array

The only entry in genericv8_cp15_regs[] is for emulation of ACTLR.
As all targets emulate this in the same way, move it to cp15_regs[].

This will let us kill off the target specific abstraction.

Signed-off-by: James Morse <james.morse@arm.com>