[WATCHDOG] support for max63xx watchdog timer chips

This driver adds support for the max63{69,70,71,72,73,74} family of
watchdog timer chips.

It has been tested on an Arcom Zeus (max6369).

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>