watchdog: Add hardware dependency to BCM7038 driver

The BCM7038 watchdog driver is specific to Broadcom ARM and MIPS
SoCs so do not present it on other architectures, unless
build-testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Justin Chen <justinpopo6@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>