mmc: split fsl_esdhc driver for i.MX

mmc: split fsl_esdhc driver for i.MX

The fsl_esdhc driver was for Freescale eSDHC on MPC83XX/MPC85XX
initially. The later QoriQ series PowerPC processors (which were
evolutions of MPC83XX/MPC85XX), QorIQ series ARM processors, and
i.MX series processors were using this driver for their eSDHCs too.

For the two series processors, the eSDHCs are becoming more and
more different. We should have split it into two drivers, like them
(sdhci-of-esdhc.c/sdhci-esdhc-imx.c) in linux kernel.

This patch is just to create a fsl_esdhc_imx driver which is a copy
of fsl_esdhc driver for i.MX processors. We will convert i.MX processors
to use fsl_esdhc_imx, and clean up the two drivers separately in the
future patches.

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