mmc: sdhci-esdhc-imx: do not reference platform data after probe

The patch copies platform data into pltfm_imx_data and reference
the data there than platform data after probe.

This work is inspired by Grant Likely and Troy Kisky.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
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>