arm: mach-mx3: use IMX_GPIO_NR instead of hard-coded values

The latter are error-prone because the bank number is one less than one
would read in the documentation.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Eric Benard <eric@eukrea.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>