mmc: zynq_sdhci: Remove check for DM_MMC config

The zynq_sdhci drivers depends on DM_MMC in Kconfig so no need to check for
DM_MMC in the driver so this patch removes it.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>