feat(sgi): enable fpregs context save and restore

This is required to prevent Nwd context corruption during StMM
execution.

Standalone MM uses OpenSSL for secure boot, which uses FP registers for
floating point calculations.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I6ed11d4fa5d64c3089a24b66fd048a841c480792