defconfig: lahaina-gki: enable stub-regulator driver

Enable the stub-regulator driver so that regulator consumers can
successfully request handles to stub-regulator devices used on
Lahaina boards.

Change-Id: I5d406a068cbd06e29f3fd6d2801779b1ad8e6564
Signed-off-by: David Collins <collinsd@codeaurora.org>