SPM: Remove unnecessary register save

Since commit 01fc1c24 ("BL31: Use helper function to save registers
in SMC handler") all the general-purpose registers are saved when
entering EL3. It isn't needed to save them here.

Change-Id: Ic540a5441b89b70888da587ab8fc3b2508cef8cc
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>