arm64: neon/efi: Make EFI fpsimd save/restore variables static

The percpu variables efi_fpsimd_state and efi_fpsimd_state_used,
used by the FPSIMD save/restore routines for EFI calls, are
unintentionally global.

There's no reason for anything outside fpsimd.c to touch these, so
this patch makes them static (as they should have been in the first
place).

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>