ARM: KVM: vgic mmio should hold data as LE bytes array in BE case

According to recent clarifications of mmio.data array meaning -
the mmio.data array should hold bytes as they would appear in
memory. Vgic is little endian device. And in case of BE image
kernel side that emulates vgic, holds data in BE form. So we
need to byteswap cpu<->le32 vgic registers when we read/write them
from mmio.data[].

Change has no effect in LE case because cpu already runs in le32.

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