ENGR00269945: pinctrl: support pinctrl setting assertion via gpios

ENGR00269945: pinctrl: support pinctrl setting assertion via gpios

It's pretty common that on some reference design or validation boards,
one pin could be used by two devices on board, and the pin route is
controlled by a GPIO.  So to assert the pin for given device, not only
the pinmux controller in SoC needs to be set up properly but also the
GPIO needs to be pulled up/down.

The patch adds support of a device tree property "pinctrl-assert-gpios"
under client device node.  It plays pretty much like a board level pin
multiplexer, and steers the pin route by controlling the GPIOs.  When
client device has the property represent in its node, pinctrl device
tree mapping function will firstly pull up/down the GPIOs to assert the
pins for the device at board level.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>