arm, Makefile: correct compilation flag for u-boot-dtb

The compilation of u-boot-dtb.img should be controlled by
CONFIG_OF_CONTROL and not CONFIG_DM.
CONFIG_DM may be defined even without Device Tree requirement.

This was added in commit 947cee11.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
Signed-off-by: Aneesh Bansal <aneesh.bansal@freescale.com>
CC: Alison Wang <alison.wang@freescale.com>