OvmfPkg/RiscVVirt: Enable CpuMmio2Dxe

CpuMmio2Dxe is supports MMIO, enable it.

Build-tested only (with "RiscVVirtQemu.dsc").

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4584

Cc: Sunil V L <sunilvl@ventanamicro.com>
Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Signed-off-by: Chao Li <lichao@loongson.cn>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

OvmfPkg/RiscVVirt/RiscVVirtQemu.dsc

@@ -433,7 +433,7 @@
   #
   # PCI support
   #
-  OvmfPkg/RiscVVirt/PciCpuIo2Dxe/PciCpuIo2Dxe.inf {
+  UefiCpuPkg/CpuMmio2Dxe/CpuMmio2Dxe.inf {
     <LibraryClasses>
       NULL|OvmfPkg/Fdt/FdtPciPcdProducerLib/FdtPciPcdProducerLib.inf
   }