watchdog: retu: restore MFD dependency

The retu watchdog calls into the respective mfd driver, but fails to
link if that is diabled:

drivers/watchdog/built-in.o: In function `retu_wdt_set_timeout':
ziirave_wdt.c:(.text+0x8c88): undefined reference to `retu_write'
ziirave_wdt.c:(.text+0x8c88): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `retu_write'
drivers/watchdog/built-in.o: In function `retu_wdt_start':
ziirave_wdt.c:(.text+0x8cc8): undefined reference to `retu_write'
ziirave_wdt.c:(.text+0x8cc8): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `retu_write'

This restores the dependency as it was before

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>