PcAtChipsetPkg AcpiTimerLib: Clear bits [31:24] after reading by IoRead32()

PcAtChipsetPkg AcpiTimerLib: Clear bits [31:24] after reading by IoRead32()

Clear bits [31:24] after reading ACPI timer count by IoRead32(), and also add
comments "Note: The implementation uses the lower 24-bits of the ACPI timer
and is compatible with both 24-bit and 32-bit ACPI timers." in INF.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>