watchdog: menf21bmc: add I2C dependency

This driver fails to link when CONFIG_I2C is disabled or a loadable module while
the watchdog is built-in:

drivers/watchdog/built-in.o: In function `menf21bmc_wdt_shutdown':
menf21bmc_wdt.c:(.text+0x9b44): undefined reference to `i2c_smbus_write_word_data'
menf21bmc_wdt.c:(.text+0x9b44): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `i2c_smbus_write_word_data'

This adds a Kconfig dependency for it, to enforce a valid configuration.

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>