feat(spm-mm): switch to simd_ctx_save/restore APIs

This patch demonstrates the trivial changes to transparently switch
the fpregs_context_* helpers to simd_ctx_* helpers.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Signed-off-by: Okash Khawaja <okash@google.com>
Change-Id: I14bda6bd0ead1f34a570b59be8dec3ac40891c20