feat(el3-spmc): synchronize access to the s-el0 sp context

This patch locks and unlocks access to the S-EL0 SP context when its
runtime state and model are updated to avoid issues around concurrent
access to global state.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I427657050574c189cbaf82c1371e3ee44bc1663e