bcma: set the pmu watchdog if available

Mostly all bcma based devices have a PMU and the PMU watchdog should be
used and not the old one in chip common. This patch also calculates the
maximal number the watchdog could be set to.

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