watchdog: wm831x watchdog really needs mfd

The wm831x watchdog driver can now be built without the wm831x mfd
driver, which results in a link error:

(.text+0x1a95c): undefined reference to `wm831x_set_bits'
(.text+0x1a95c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `wm831x_set_bits'
(.text+0x1a968): undefined reference to `wm831x_reg_lock'
(.text+0x1a968): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `wm831x_reg_lock'
(.text+0x1a9dc): undefined reference to `wm831x_reg_unlock'
(.text+0x1a9dc): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `wm831x_reg_unlock'

This adds back the dependency that was removed. We can still build test
this driver on all architectures by enabling the MFD driver for it first.

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>