ARM: zynq_defconfig: Remove debug UART

ARM: zynq_defconfig: Remove debug UART

As the help text points out:
"Note that selecting this option will limit the kernel to a single
UART definition, as specified below. Attempting to boot the kernel
image on a different platform *will not work*, so this option should
not be enabled for kernels that are intended to be portable."

To avoid hangs on systems that don't use the expected UART, remove this
from the defconfig.

Reported-by: Andrei Simion <andreis@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>