arm64: move FP-SIMD save/restore code to a macro

In order to be able to reuse the save-restore code in KVM, move
it to a pair of macros, similar to what the 32bit code does.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>