pinctrl: zynq: Add a 8 bit wide nand option

The hardware supports a 16 and 8 bit wide NAND bus, let users pick
either.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>