defconfig: am335x: bbb: enable mmc driver model

enable mmc driver model for am335x bbb as omap_hsmmc
supports driver model

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

configs/am335x_boneblack_vboot_defconfig

@@ -14,3 +14,4 @@ CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT,ENABLE_VBOOT"
 CONFIG_OF_CONTROL=y
 CONFIG_SPI_FLASH=y
 CONFIG_DM_ETH=y
+CONFIG_DM_MMC=y