watchdog: kempld: revert to full dependency

The kempld watchdog driver requires the respective MFD driver:

drivers/watchdog/built-in.o: In function `kempld_wdt_probe':
kempld_wdt.c:(.text+0x5c78): undefined reference to `kempld_get_mutex'
kempld_wdt.c:(.text+0x5c84): undefined reference to `kempld_read8'
kempld_wdt.c:(.text+0x5c8e): undefined reference to `kempld_release_mutex'
kempld_wdt.c:(.text+0x5d1c): undefined reference to `kempld_read8'
kempld_wdt.c:(.text+0x5d2c): undefined reference to `kempld_write8'

This adds the Kconfig dependency back.

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>