Do not hardcode the register region to SZ_4K as this is not correct for mx31.

Use data->iosize instead which works for both mx27 and mx31 cases.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>