arm64: Remove unnecessary ISB when writing to SPSel

Software can use the SPSel operand to write directly to PSTATE.SP.
According to ARM DDI 0487F.b, page D1-2332, writes to PSTATE are
self-synchronizing and no ISB is needed:

"Writes to the PSTATE fields have side-effects on various aspects of the PE
operation. All of these side-effects are guaranteed:
- Not to be visible to earlier instructions in the execution stream.
- To be visible to later instructions in the execution stream."

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>