mmc: dw_mmc: Indicate that regulators may be absent

Use regulator_get_optional() to tell the core that requests for regulators
can fail in a real system.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Chris Ball <cjb@laptop.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>