mmc: mxcmmc: constify mxcmci_devtype

mxcmci_devtype struct contains constant data, so
constify this struct.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>