ARM: sunxi: Add support for R_PIO gpio banks

ARM: sunxi: Add support for R_PIO gpio banks

The A31, A23 and later SoCs have an extra pin controller, called CPUs_PIO
or R_PIO, which handles pin banks L and beyond.

Also add a clear description about SUNXI_GPIO_BANKS, stating it only
counts the number of pin banks in the _main_ pin controller.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[wens@csie.org: expanded commit message]
[wens@csie.org: add pin bank M and expand comments]
[wens@csie.org: add comment on SUNXI_GPIO_BANKS macro]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>