Use FixedPCD's to set platform-specific values for RTC registers.
Specifically, the replaced macros are:
1) RTC_INIT_REGISTER_A
2) RTC_INIT_REGISTER_B
3) RTC_INIT_REGISTER_D
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leo Duran <leo.duran@amd.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>