pca100: clean up GPIO pin multiplexing

cleaned up some duplicated mxc_gpio_mode calls and move the correpsonding
pin multiplexing either into the initial pin config table or ifdef blocks.

Signed-off-by: Luotao Fu <l.fu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>