ssb/bcma: add common header for watchdog

This adds a common header for watchdog functions, so a watchdog driver
just needs to use this and could provide watchdog functionality for ssb
and bcma based SoCs. Patches for a watchdog driver using this interface
will be send later.

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