KVM: doc: Add API documentation on the KVM_REG_ARM_WORKAROUNDS register

Add documentation for the newly defined firmware registers to save and
restore any vulnerability mitigation status.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>