allwinner: Export sunxi_private.h

allwinner: Export sunxi_private.h

So far we have a sunxi_private.h header file in the common code directory.
This holds the prototypes of various functions we share in *common*
code. However we will need some of those in the platform specific code
parts as well, and want to introduce new functions shared across the
whole platform port.

So move the sunxi_private.h file into the common/include directory, so
that it becomes visible to all parts of the platform code.
Fix up the existing #includes and add missing ones, also add the
sunxi_read_soc_id() prototype here.

This will be used in follow up patches.

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