watchdog: bcm63xx_wdt: fix driver section mismatch

bcm63xx_wdt was used as a platform_driver but was not suffixed with
_driver, thus causing section mismatches, fix that.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>