sunxi_nand_spl: Add support for sun4i and sun5i SoCs

Other then having a few less chip-select lines the nand controller
on sun4i, sun5i and sun7i is identical.

Note this patch also muxes GPC7 to the NAND on sun7i where as before
it was not muxed this way. GPC7 is a standard NAND pin, so it should
always be muxed to the NAND when in use.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>