PcAtChipsetPkg PcRtc: Use new EfiLocateFirstAcpiTable()

https://bugzilla.tianocore.org/show_bug.cgi?id=967


Request to add a library function for GetAcpiTable() in order
to get ACPI table using signature as input.

After evaluation, we found there are many duplicated code to
find ACPI table by signature in different modules.

This patch updates PcatRealTimeClockRuntimeDxe to use new
EfiLocateFirstAcpiTable() and remove the duplicated code.

Cc: Younas khan <pmdyounaskhan786@gmail.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>