OvmfPkg/AcpiPlatformDxe: Consume the 8259 PCD defined in OvmfPkg

OvmfPkg/AcpiPlatformDxe: Consume the 8259 PCD defined in OvmfPkg

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1496

Several updates have been made to the OvmfPkg/AcpiPlatformDxe driver to
drop its dependency on PcAtChipsetPkg:

A) Consumes the PCD 'Pcd8259LegacyModeEdgeLevel' defined within OvmfPkg;
B) Remove the PcAtChipsetPkg DEC file dependency in the driver INF file.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>