This new implemenatation avoids that two physical pins are claimed by
the same driver (also with the the gpr hardware modes).
The gpio kernel lib is also called when a capable gpio pin is assigned
its gpio function.
The mxc_iomux_mode function is still here for backward compatibility but
should not be used anymore.

V2:

In the precendent revision, the iomux code was claiming a pin when its
hardware mode was changed. This was uncorrect: when the hardware mode is
changed, the pin must still be claimed through the iomux.

In order to have a pin working in mode hw2, we must fist issue the
mxc_iomux_set_gpr call and then the corresponding mxc_iomux_mode calls
with the FUNC mode (usually done with mxc_iomux_setup_multiple_pins).
The reverse calls must be done to fee the pins.
Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>