[ARM] MX1/MX2: simplify mxc_gpio_setup_multiple_pins

mxc_gpio_setup_multiple_pins used to take several ALLOC_MODE flags. Most
of them are unused, so simplify the function by removing the flags. Also,
instead of using a confusing MXC_GPIO_ALLOC_MODE_RELEASE flag in a function
having alloc in its name, add a mxc_gpio_release_multiple_pins function.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>