ArmPlatformPkg: eliminate Juno gcc build warning

When building without LTO, gcc incorrectly resolves the hazards for
'PciRegBase’ when inlining, leading to "may be used uninitialized"
warnings (and hence build failure with -Werror).
Eliminate this warning by explicitly initializing the variable to 0.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>

ArmPlatformPkg/ArmJunoPkg/Drivers/ArmJunoDxe/ArmJunoDxe.c

@@ -343,6 +343,7 @@ ArmJunoSetNicMacAddress ()
     return Status;
   }
 
+  PciRegBase = 0;
   Status = InitPciDev (PciIo, &PciRegBase, &OldPciAttr);
   if (EFI_ERROR (Status)) {
     return Status;