drivers: qcom: rpmh-rsc: modularize RSC controller driver

Enable RSC controller driver to be compiled as modules. Since RPMH
communication is need for enabling/disabling critical clocks and busses
needed for the application processor, it is better to keep this module
enabled. Hence declare the driver as a builtin platform driver.

Signed-off-by: Hridya Valsaraju <hridya@google.com>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>

Change-Id: I5e90b68bbf2c7ed3a2666e0687ff4ade40b9c7a3