rtc: bd70528: fix BD71815 watchdog dependency

The added Kconfig dependency is slightly incorrect, which can
lead to a link failure when the watchdog is a loadable module:

arm-linux-gnueabi-ld: drivers/rtc/rtc-bd70528.o: in function `bd70528_set_rtc_based_timers':
rtc-bd70528.c:(.text+0x6cc): undefined reference to `bd70528_wdt_set'
arm-linux-gnueabi-ld: drivers/rtc/rtc-bd70528.o: in function `bd70528_set_time':
rtc-bd70528.c:(.text+0xaa0): undefined reference to `bd70528_wdt_lock'
arm-linux-gnueabi-ld: rtc-bd70528.c:(.text+0xab8): undefined reference to `bd70528_wdt_unlock'
arm-linux-gnueabi-ld: drivers/rtc/rtc-bd70528.o: in function `bd70528_alm_enable':
rtc-bd70528.c:(.text+0xfc0): undefined reference to `bd70528_wdt_lock'
arm-linux-gnueabi-ld: rtc-bd70528.c:(.text+0x1030): undefined reference to `bd70528_wdt_unlock'

The problem is that it allows to be built-in if MFD_ROHM_BD71828
is built-in, even when the watchdog is a loadable module.

Rework this so that having the watchdog as a loadable module always
forces the rtc to be a module as well instead of built-in,
regardless of bd71828.

Fixes: c56dc069

 ("rtc: bd70528: Support RTC on ROHM BD71815")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210422151545.2403356-1-arnd@kernel.org