mmc: sdhci-esdhc-imx: get rid of the uses of cpu_is_mx()

The patch removes all the uses of cpu_is_mx().  Instead, it utilizes
platform_device_id to distinguish the esdhc differences among SoCs.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Cc: Chris Ball <cjb@laptop.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Chris Ball <cjb@laptop.org>