PcAtChipsetPkg: Move RTC PCD to dynamic PCD
    
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4193
    
    In order to remove RTC_INDEX/RTC_TARGET from
    the UplBuild macro list,change the RTC_INDEX
    /RTC_TARGET type from PcdsFixedAtBuild to PcdsDynamicEx
    
    Cc: Guo Dong <guo.dong@intel.com>
    Reviewed-by: Ray Ni <ray.ni@intel.com>
    Cc: James Lu <james.lu@intel.com>
    Reviewed-by: Gua Guo <gua.guo@intel.com>
    Signed-off-by: KasimX Liu <kasimx.liu@intel.com>