ssb: set the PMU watchdog if available

Some ssb based devices have a PMU and the PMU watchdog register should
be used instead of the register in the chip common part, if the device
has a PMU. 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>