ARM: mxc: use ARCH_NR_GPIOS to define gpio number

The patch removes MXC_GPIO_IRQS and instead uses ARCH_NR_GPIOS to
define gpio number.  This change is need when we change mxc gpio
driver to be device tree aware.  When migrating the driver to device
tree, pdev->id becomes unusable.  It requires driver get gpio range
from gpio core, which will dynamically allocates number from
ARCH_NR_GPIOS to 0.

As a bonus point, it removes lines of '#if' and make the code a
little bit cleaner.  The side effect is the waste of number.  But
this is not a point when we go single image.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>