rdn2 changes and adding support for new variant rdn2cfg3
Created by: tonykn-arm
Patches in this pull request
- code refactoring to use PLATFORM_VARIANT build option instead of NUMBER_OF_CLUSTERS macro
- change REFCLK clock frequency to 125MHz to support increment the system counter at a fixed frequency of 1GHz
- Adding support for new RD-N2 variant (rdn2cfg3)