mmc: fsl_esdhc: drop i.MX DDR support code

A previous patch below adding DDR mode support was actually for i.MX
platforms. Now i.MX eSDHC driver is fsl_esdhc_imx.c. For QorIQ eSDHC,
it uses different process for DDR mode, and hasn't been supported.
Let's drop DDR support code for i.MX in fsl_esdhc driver.

0e1bf614 mmc: fsl_esdhc: Add support for DDR mode

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>