watchdog: bcm47xx_wdt.c: allow enabling on BCM5301X arch

BCM5301X (ARCH_BCM_5301X) is a new Broadcom architecture using the same
SoC bus driver (bcma) as BCM47XX but based on ARM instead of MIPS.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>