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 <[email protected]>
[[email protected]: expanded commit message]
[[email protected]: add pin bank M and expand comments]
[[email protected]: add comment on SUNXI_GPIO_BANKS macro]
Signed-off-by: Chen-Yu Tsai <[email protected]>
Acked-by: Ian Campbell <[email protected]>