[ARM] MXC: Fix mxc_gpio_get(), which must read PSR register instead DR.

The Data register holds the value we have written to a gpio. To
get the input value we must read the Pad Status Register MX3 (or Sample
Status register in MX1/2 terms)

Signed-off-by: Darius Augulis <augulis.darius@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>