ARM: dts: imx50: make pinctrl nodes board specific

ARM: dts: imx50: make pinctrl nodes board specific

Currently, all pinctrl setting nodes are defined in <soc>.dtsi, so that
boards that share the same pinctrl setting do not have to define it time
and time again in <board>.dts.  However, along with the devices and use
cases being added continuously, the pinctrl setting nodes under iomuxc
becomes more than expected.  This bloats device tree blob for particular
board unnecessarily since only a small subset of those pinctrl setting
nodes will be used by the board.  It impacts not only the DTB file size
but also the run-time device tree lookup efficiency.

The patch provides a solution to avoid this device tree bloating problem
while still keeping boards share the common pinctrl setting data by
using DTC macro support.  It creates <soc>-pingrp.h and move all those
pinctrl setting data into there as macro definitions.  The <board>.dts
will instead define the pinctrl setting nodes that are necessary for the
board by referring to the macros in <soc>-pingrp.h, so that only the
pinctrl setting data that will be used by the board will get compiled
into the DTB for the board.

With the changes, the pinctrl setting nodes becomes local to particular
board, and it makes no sense to continue numbering the setting for
given peripheral.  Thus, all the pinctrl phandler name gets updated to
have only peripheral name in there.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>