[REFACTOR] Force the SMD regulator driver to be compiled-in

Allowing REGULATOR_QCOM_SMD_RPM to be a module requires solving exporting
some symbols and fixing some dependencies. Since this driver is due to be
refactored, let's take a shortcut to simply disallow modules for now.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>