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.

Change-Id: I41a657c0d618efb7c3460833ffb8eb5d445885a0
Signed-off-by: Lina Iyer <ilina@codeaurora.org>