mfd: STw481x: Allow modular build

This driver depends on I2C, which may be a loadable module.
While you'd probably want both to be built-in in practice,
allowing a modular build avoids possible randconfig link
errors.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>