SourceLevelDebugPkg/DebugCommLibUsb3Pei: Make sure alloc physical mem

PI 1.6 has supported pre permanent memory page allocation,
to make sure the allocated memory is physical memory for DMA,
the patch is to check memory discovered PPI installed or not first
before calling AllocatePages.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
(cherry picked from commit 364f63c065)

SourceLevelDebugPkg/Library/DebugCommunicationLibUsb3/DebugCommunicationLibUsb3Pei.inf

@@ -1,7 +1,7 @@
 ## @file
 #  Debug Communication Library instance based on usb3 debug port.
 #
-#  Copyright (c) 2014 - 2015, Intel Corporation. All rights reserved.<BR>
+#  Copyright (c) 2014 - 2017, Intel Corporation. All rights reserved.<BR>
 #
 #  This program and the accompanying materials
 #  are licensed and made available under the terms and conditions of the BSD License
@@ -40,6 +40,9 @@
   IntelFrameworkPkg/IntelFrameworkPkg.dec
   SourceLevelDebugPkg/SourceLevelDebugPkg.dec
 
+[Ppis]
+  gEfiPeiMemoryDiscoveredPpiGuid                ## CONSUMES
+
 [Pcd]
   # The memory BAR of ehci host controller, in which usb debug feature is enabled.
   # Note that the memory BAR address is only used before Pci bus resource allocation.