[ARM] pxa/spitz: use generic GPIO API and remove pxa_gpio_mode()

REVISIT: change to GPIO18 is ugly, need to make sure whether that's
really necessary - GPIO18_RDY as an VLIO input signal - we don't
normally need to do such kind of trick during low power mode.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>