defconfig: Enable stub regulator for sdxpoorwills

Stub regulators allows consumers to specify valid
regulator handles.

Change-Id: Ie518a4394b57a8189065206ad26d859a18068ea6
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Signed-off-by: Darshan Shah <cdarshn@codeaurora.org>