ARM64: KVM: set and get of sys registers in BE case

Since size of all sys registers is always 8 bytes. Current
code is actually endian agnostic. Just clean it up a bit.
Removed comment about little endian. Change type of pointer
from 'void *' to 'u64 *' to enforce stronger type checking.

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>