ARM: configs: disable DEBUG_LL in bcm_defconfig

DEBUG_LL is not supported yet on bcm281xx and causes a temporary
hang due to timeout at boot.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Christian Daudt <csd@broadcom.com>