mmc: fsl_esdhc: drop i.MX code

Dropped i.MX code which couldn't be reused.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Tested-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Martyn Welch <martyn.welch@collabora.com>