gpio: mxs: remove useless function

Fix the following gcc warning:

drivers/gpio/gpio-mxs.c:63:19: warning: kernel/sys_ni.cunused function
'is_imx28_gpio'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>