defconfig: sdxprairie: enable stub-regulator driver

Enable the stub-regulator driver so that regulator consumers
can successful request handles to PMXPRAIRIE stub-regulator
devices.

Change-Id: I94afd5d6089c6c66dd68dd5b38d6b4f6f0549b99
Signed-off-by: David Collins <collinsd@codeaurora.org>