defconfig: lahaina-gki: enable the fixed regulator driver

Enable the fixed regulator driver so that consumers can control
fixed regulator devices at runtime.  These include both GPIO
controlled regulators and always-on (non-controllable) board
regulators.

Change-Id: I599c48e825fe546219146a893b5c8101a7e681ca
Signed-off-by: David Collins <collinsd@codeaurora.org>