PcAtChipsetPkg/PcRtc: Add two new PCD for RTC Index/Target registers

PcAtChipsetPkg/PcRtc: Add two new PCD for RTC Index/Target registers

In certain HW implementation, the BIT7 of RTC Index register(0x70) is
for NMI sources enable/disable but the BIT7 of 0x70 cannot be read
before writing. Software which doesn't want to change the NMI sources
enable/disable setting can write to the alias register 0x74, through
which only BIT0 ~ BIT6 of 0x70 is modified.
So two new PCDs are added so that platform can have the flexibility
to change the default RTC register addresses from 0x70/0x71 to
0x74/0x75.
With the new PCDs added, it can also support special HW that provides
RTC storage in a different register pairs.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>