allwinner: Detect and output current SoC

So far we already support booting on two different SoCs, and we will
shortly add a third, so add some code to determine the current SoC type.
This can be later used to runtime detect certain properties.

Also print the SoC name to the console, to give valuable debug information.

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