ARM: KVM: Add banked registers save/restore

Banked registers are one of the many perks of the 32bit architecture,
and the world switch needs to cope with it.

This requires some "special" accessors, as these are not accessed
using a standard coprocessor instruction.

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