imx: dynamic setting mmcdev and mmcroot

Dynamic setting mmcdev and mmcroot.
Then when boot linux, we can have correct "root=/dev/mmcblk[x]p2"

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/common/Makefile b/board/freescale/common/Makefile
index f13965d..4df4849 100644
--- a/board/freescale/common/Makefile
+++ b/board/freescale/common/Makefile
@@ -63,6 +63,9 @@
 obj-$(CONFIG_POWER_PFUZE100)	+= pfuze.o
 obj-$(CONFIG_DM_PMIC_PFUZE100)	+= pfuze.o
 obj-$(CONFIG_POWER_MC34VR500)	+= mc34vr500.o
+ifneq (,$(filter $(SOC), imx8ulp))
+obj-y				+= mmc.o
+endif
 
 obj-$(CONFIG_LS102XA_STREAM_ID)	+= ls102xa_stream_id.o