Change MMC device name to match kernel device tree.

http://b/63889826

Change-Id: I3490a6bb9e5838a06275e83922e4cad1cf26aaaf
diff --git a/drivers/mmc/bcm2835_sdhci.c b/drivers/mmc/bcm2835_sdhci.c
index 08bddd4..b117c27 100644
--- a/drivers/mmc/bcm2835_sdhci.c
+++ b/drivers/mmc/bcm2835_sdhci.c
@@ -228,7 +228,7 @@
 }
 
 static const struct udevice_id bcm2835_sdhci_match[] = {
-	{ .compatible = "brcm,bcm2835-sdhci" },
+	{ .compatible = "brcm,bcm2835-mmc" },
 	{ /* sentinel */ }
 };