mmc: fsl_esdhc: fix probe issue without CONFIG_BLK enabled

u-boot is trying to make CONFIG_BLK as a hard requirement
for DM_MMC. But now it's still not.

config BLK
	bool "Support block devices"
	depends on DM
	default y if DM_MMC

When fsl_esdhc driver was reworked for DM_MMC support, DM_MMC
without CONFIG_BLK enabled wasn't considered. This patch is to
fix probe issue without CONFIG_BLK enabled.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Yinbo Zhu <yinbo.zhu@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>