mx31: remove gpio_request calls from iomux code

Since iomux code is not directly related to gpio on mx31, the calls
to gpio_request are removed from iomux.c file.

These calls have to be done in platform initialization files. The
name of the singe pin call for iomux is also changed to
mxc_iomux_alloc_pin.

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>