watchdog: bcm2835: add CONFIG_OF dependency

Without CONFIG_OF, the driver fails to link:

drivers/watchdog/built-in.o: In function `bcm2835_power_off':
bcm2835_wdt.c:(.text+0x1946): undefined reference to `of_find_device_by_node'

This adds a new dependency, to allow the COMPILE_TEST check to succeed.

Fixes: da2a68b3

 ("watchdog: Enable COMPILE_TEST where possible")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>