docs(allwinner): update SoC list and build options

Our list of possible Allwinner build targets was missing the newly
introduced R329 support. Fix that by adding a table with maps the SoC
names to the build target names.
Also add some explanation about the recently introduced PSCI power
management providers.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Icf5e304562c3082552bf08d7b26904caf9074936
1 file changed