ARM: multi_v7_defconfig: enable ARM_ATAG_DTB_COMPAT

Without this, legacy platforms that can boot with a multiplatform
kernel but that need the DTB to be appended, won't have a way to pass
firmware-set bootargs to the kernel.

This is needed to boot multi_v7_defconfig on snowball, for instance.

Signed-off-by: Olof Johansson <olof@lixom.net>