On MX1, MX21 and MX27 each GPIO port has an address space of 256 bytes.

Fix the iosize for these platforms.

Tested on a mx27_3ds board that can boot fine after this change.

Cc: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>