ArmVirtPkg/FdtClientDxe: report address and size cell count directly

The FDT client protocol methods dealing with "reg" properties return
the size of a "reg" element. Currently, we have hardcoded this as '8',
since #address-cells == #size-cells == 2 in most cases. However, for
different values, have a single 'reg' element size is not unambiguous,
since - however unlikely - if #address-cells != #size-cells, we do not
know which is which.

So before adding more methods to the protocol, fix up this oversight.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

ArmVirtPkg/XenAcpiPlatformDxe/XenAcpiPlatformDxe.c

@@ -45,7 +45,8 @@ GetXenArmAcpiRsdp (
   EFI_STATUS                                     Status;
   FDT_CLIENT_PROTOCOL                            *FdtClient;
   CONST UINT64                                   *Reg;
-  UINT32                                         RegElemSize, RegSize;
+  UINT32                                         RegSize;
+  UINTN                                          AddressCells, SizeCells;
   UINT64                                         RegBase;
   UINT8                                          Sum;
 
@@ -59,13 +60,16 @@ GetXenArmAcpiRsdp (
   ASSERT_EFI_ERROR (Status);
 
   Status = FdtClient->FindCompatibleNodeReg (FdtClient, "xen,guest-acpi",
-                        (CONST VOID **)&Reg, &RegElemSize, &RegSize);
+                        (CONST VOID **)&Reg, &AddressCells, &SizeCells,
+                        &RegSize);
   if (EFI_ERROR (Status)) {
     DEBUG ((EFI_D_WARN, "%a: No 'xen,guest-acpi' compatible DT node found\n",
       __FUNCTION__));
     return EFI_NOT_FOUND;
   }
 
+  ASSERT (AddressCells == 2);
+  ASSERT (SizeCells == 2);
   ASSERT (RegSize == 2 * sizeof (UINT64));
 
   RegBase = SwapBytes64(Reg[0]);