gpio: mxs: read pin level directly instead of using .get

Calling readl directly instead of going through another function that
results in the same result to remove some overhead. I didn't try to
measure the performance gain, but IMHO there is little benefit from
abstracting a GPIO register access in the GPIO driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>