ipq806x: enable watchdog kernel option

Some bootloaders seem to trigger the watchdog during the boot process,
therefore the lack of watchdog driver trigger a reboot a few seconds
after boot. So we'll enable it here to avoid it.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44503 3c298f89-4303-0410-b956-a3cf2f4a3e73