ARM: zynq: Do not enable NETBSD support by default

This option is disable in Xilinx tree for quite a long time that's why
let's disable it in mainline. If there is anybody who requires this options
then it should be moved to Kconfig first and should be removed from this
config.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>