uboot-imx

imx: mx7dsabresd: support nand

Add pinmux settings.
Add related macro definitions.
Nand pin conflicts with emmc, so if want to enable nand, need to do
hardware rework. After hardware rework, define CONFIG_NAND_MXS in
board header file.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>