dm: mmc: Set up the device pointer when using the MMC uclass

Update the existing drivers to set up this new pointer. This will be required
by the MMC uclass.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/zynq_sdhci.c b/drivers/mmc/zynq_sdhci.c
index b59feca..d405929 100644
--- a/drivers/mmc/zynq_sdhci.c
+++ b/drivers/mmc/zynq_sdhci.c
@@ -35,6 +35,7 @@
 		  CONFIG_ZYNQ_SDHCI_MIN_FREQ);
 
 	upriv->mmc = host->mmc;
+	host->mmc->dev = dev;
 
 	return 0;
 }