armv8: Force SP_ELx stack pointer usage

In ARMv8 we have the choice between two stack pointers to use: SP_EL0 or
SP_ELx, which is banked per exception level. This choice is stored in
the SP field of PState, and can be read and set via the SPSel special
register. When the CPU takes an exception, it automatically switches to
the SP_ELx stack pointer.

Trusted Firmware enters U-Boot typically with SPSel set to 1, so we use
SP_ELx all along as our sole stack pointer, both for normal operation and
for exceptions.

But if we now for some reason enter U-Boot with SPSel cleared, we will
setup and use SP_EL0, which is fine, but leaves SP_ELx uninitialised.
When we now take an exception, we try to save the GPRs to some undefined
location, which will usually end badly.

To make sure we always have SP_ELx pointing to some memory, set SPSel
to 1 in the early boot code, to ensure safe operation at all times.

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