efi/arm: add SIMD stash/unstash operations

This adds the SIMD stash/unstash operations that we need to allow UEFI
runtime services calls to call back into the kernel. In the ARM case,
these are actually NOPs, since UEFI on ARM is not allowed to use the
FP/NEON register file.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>