ARM: u300: configure some pins as an example

To show how the pin configuration is used on the U300, let's
include some configs for two GPIO pins.

ChangeLog v1->v2:
- Provide a better example, set the clock return pin to pull-up
  and set the card detect pin to high impedance.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>