mmc: mxs-mmc: add non-removable property

Some boards have non removable cards like eMMC. Handle such case.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>