ssb: register watchdog driver

Register the watchdog driver to the system if it is a SoC. Using the
watchdog on a non SoC device, like a PCI card, will make the PCI
card die when the timeout expired, but starting it again is not
supported by ssb.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>