fix(spe): drop SPE EL2 context switch code

At the moment we hardcode the SPE functionality to be available on the
non-secure side only, by setting MDCR_EL3.NSPB accordingly.
This also means that the secure world cannot use SPE, so there is no
need to context switch the PMSCR_EL2 register.

Drop the SPE bits from the EL2 context switch code. If any of the other
EL2 worlds wish to start using SPE, this can be brought back.

Change-Id: Ie0fedb2aeb722a2c9db316051fbbe57ca0e3c0c9
Signed-off-by: Andre Przywara <andre.przywara@arm.com>