allwinner: Allow conditional compilation of SCPI and native PSCI ops

Now that we have split the native and the SCPI version of the PSCI ops,
we can introduce build options to compile in either or both of them.

If one version is not compiled in, some stub functions make sure the
common code still compiles and makes the right decisions.

By default both version are enabled (as before), but one of them can be
disabled on the make command line, or via a platform specific Makefile.

Change-Id: I0c019d8700c0208365eacf57809fb8bc608eb9c0
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>