ARM: mxs: add helper macro for pad control

This patch is to add pad control helper macro to make the code easy
to read.  The need is being seen when adding pad definitions for
LCDIF which gets ~30 pads to define.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>