[WATCHDOG] add bcm47xx watchdog driver

This add watchdog driver for broadcom 47xx device.
It uses the ssb subsytem to access embeded watchdog device.

Because the watchdog timeout is very short (about 2s), a soft timer is used
to increase the watchdog period.

Note : A patch for exporting the ssb_watchdog_timer_set will
be submitted on next linux-mips merge. Without this patch it can't 
be build as a module.

Signed-off-by: Aleksandar Radovanovic <biblbroks@sezampro.rs>
Signed-off-by: Matthieu CASTET <castet.matthieu@free.fr>
Tested-by: Florian Fainelli <florian@openwrt.org>
Cc: Ralf Baechle <ralf@linux-mips.org>                                                                                       
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>