[WATCHDOG] Add support for SB1 hardware watchdog

Support watchdog timers built into SiByte MIPS SoCs.

Signed-off-by: Andy Sharp <andy.sharp@onstor.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>