ARM64: KVM: fix big endian issue in access_vm_reg for 32bit guest

Fix issue with 32bit guests running on top of BE KVM host.
Indexes of high and low words of 64bit cp15 register are
swapped in case of big endian code, since 64bit cp15 state is
restored or saved with double word write or read instruction.

Define helper macro to access low words of 64bit cp15 register.

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