gpio/mxs: remove irq_high related implementation

The irq_high stuff was mistakenly copied from mxc gpio driver.
The mxs gpio controller has only one irq line for each 32-pin
gpio port.  The patch remove irq_high related codes.

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