mmc: mxs-mmc: get rid of the use of cpu_is_xxx

The register HW_SSP_VERSION is broken for ssp version detection,
as the address of the register is different between imx23 and imx28.
Let's use platform_device_id to detect the device, so that the use
of cpu_is_xxx can be removed.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Chris Ball <cjb@laptop.org>