mmc: sdhci-esdhc-imx: Constify driver data

Variant specific driver data doesn't change at run-time, so mark it as
const to reflect that.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>