pinctrl: pxa: pxa2xx: add pin muxing

The driver is inspired from the sunxi driver. The pxa architecture
specificities leading to the driver are :
 - each pin has 8 possible alternate functions
 - 4 of these are output kind
 - 4 of these are input kind
 - there is always a "gpio input" and "gpio output" function
 - the function matrix is very scattered :
   - some functions can be found on 5 different pads
   - the number of functions is greater than the number of pins
   - there is no "topology" grouping of pins (such as all SPI in one
     corner of the die)

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>