exynos5250: config: disable max77686 driver

This PMIC is not common for all Exynos5250
based boards, so should be romoved from
common config.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>