mmc: fsl_esdhc: make BLK as hard requirement of DM_MMC

U-boot prefers DM_MMC + BLK for MMC. Now eSDHC driver has already
support it, so let's force to use it.

- Drop non-BLK support for DM_MMC introduced by below patch.
  66fa035b mmc: fsl_esdhc: fix probe issue without CONFIG_BLK enabled

- Support only DM_MMC + BLK (assuming BLK is always enabled for DM_MMC).

- Use DM_MMC instead of BLK for conditional compile.

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