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>

arch/arm/configs/multi_v7_defconfig

@@ -46,6 +46,7 @@ CONFIG_ARCH_ZYNQ=y
 CONFIG_SMP=y
 CONFIG_HIGHPTE=y
 CONFIG_ARM_APPENDED_DTB=y
+CONFIG_ARM_ATAG_DTB_COMPAT=y
 CONFIG_NET=y
 CONFIG_UNIX=y
 CONFIG_INET=y