ARM: bcm_defconfig: Do not expect appended DTB

The bootloaders used with Broadcom mobile SoCs are capable of handling
a device tree separately from the zImage so there is no need for this
option to be enabled.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>