pinctrl: add a driver for the U300 pinmux

pinctrl: add a driver for the U300 pinmux

This adds a driver for the U300 pinmux portions of the system
controller "SYSCON". It also serves as an example of how to use
the pinmux subsystem. This driver also houses the platform data
for the only supported platform.

This deletes the old U300 driver in arch/arm/mach-u300 and
replace it with a driver using the new subsystem.

The new driver is considerably fatter than the old one, but it
also registers all 467 pins of the system and adds the power
and EMIF pin groups and corresponding functions. The idea
is to use this driver as a a reference for other
implementation so it needs to be as complete and verbose
as possible.

Reviewed-by: Barry Song <21cnbao@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>