pico-imx6ul: Convert to DM MMC

Select CONFIG_DM_MMC=y in order to support MMC driver model.

This allows the MMC board related code to be removed.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/configs/pico-pi-imx6ul_defconfig b/configs/pico-pi-imx6ul_defconfig
index 62af6d8..b44be6f 100644
--- a/configs/pico-pi-imx6ul_defconfig
+++ b/configs/pico-pi-imx6ul_defconfig
@@ -42,6 +42,7 @@
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_FASTBOOT_CMD_OEM_FORMAT=y
+CONFIG_DM_MMC=y
 CONFIG_FSL_ESDHC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y