gpio: mxc_gpio: change gpio index for i.MX8

Since the i.MX8 GPIO banks are indexed from 0 not 1 on other i.MX
platforms, so we have to adjust the index accordingly.

Signed-off-by: Adrian Alonso <adrian.alonso@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>