watchdog: add support for Broadcom BCM63xx built-in watchdog

This patch adds support for the Broadcom BCM63xx SoC built-in watchdog, it
uses one of the BCM63xx hardware timer id.

Signed-off-by: Miguel Gaio <miguel.gaio@efixo.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>