regulator: Kconfig: Add Kconfig for stub regulators

Stub regulators does not perform hardware transactions with the
regulator hardware. Stub regulators allow the regulator consumer drivers
to successfully pass through the regulator callbacks

Change-Id: I3ca1d9d4b1e45ac5d545d6e39cb6dfd8887240cd
Signed-off-by: Ravindranath Thiyagarajan <rthiyaga@codeaurora.org>