bcma: register watchdog driver

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

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