watchdog: iTCO: fix link error

When the MFD driver is a loadable module, the watchdog driver fails
to get linked into the kernel:

drivers/watchdog/iTCO_wdt.o: In function `update_no_reboot_bit_pmc':
iTCO_wdt.c:(.text+0x54f): undefined reference to `intel_pmc_gcr_update'

The code is written to support operation without the MFD driver, so
add a Kconfig dependency that allows this, while disallowing the watchdog
to be built-in when the MFD driver is a module.

Fixes: 25f1ca31 ("platform/x86: intel_pmc_ipc: Convert to MFD")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20200428212959.2993304-1-arnd@arndb.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>