dm:gpio:mxc add a bank_index entry in platdata

Add a new entry in platdata structure and intialize
bank_index in mxc_plat array.
This new entry can avoid using `plat - mxc_plat` by using
`plat->bank_index`.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Simon Glass <sjg@chromium.org>