rtc: bd70528: fix driver dependencies

rtc: bd70528: fix driver dependencies

With CONFIG_BD70528_WATCHDOG=m, a built-in rtc driver cannot call
into the low-level functions that are part of the watchdog module:

drivers/rtc/rtc-bd70528.o: In function `bd70528_set_time':
rtc-bd70528.c:(.text+0x22c): undefined reference to `bd70528_wdt_lock'
rtc-bd70528.c:(.text+0x2a8): undefined reference to `bd70528_wdt_unlock'
drivers/rtc/rtc-bd70528.o: In function `bd70528_set_rtc_based_timers':
rtc-bd70528.c:(.text+0x50c): undefined reference to `bd70528_wdt_set'

Add a Kconfig dependency which forces RTC to be a module if watchdog is a
module. If watchdog is not compiled at all the stub functions for watchdog
control are used. compiling the RTC without watchdog is fine.

Fixes: 32a4a4ebf768 ("rtc: bd70528: Initial support for ROHM bd70528 RTC")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/84462e01e43d39024948a3bdd24087ff87dc2255.1565591387.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>