MIPS: ath79: use dynamically allocated watchdog device

Remove the static watchdog device variable and use
the 'platform_device_register_simple' helper to
allocate and register the device in one step.

This allows us to save a few bytes in the kernel image.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>