window interface: add close method

Add a close method, that when called does whatever is appropriate for
the given implementation.

Note: There are a few methods that don't map nicely, and therefore it
makes sense to add a close() to this interface.

Change-Id: If001446b2248715ea76147ea35a99a729ba5a404
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/lpc_nuvoton.cpp b/lpc_nuvoton.cpp
index 83b0160..b4e5477 100644
--- a/lpc_nuvoton.cpp
+++ b/lpc_nuvoton.cpp
@@ -41,6 +41,10 @@
     return std::make_unique<LpcMapperNuvoton>(regionAddress);
 }
 
+void LpcMapperNuvoton::close()
+{
+}
+
 /*
  * The host buffer address is configured by host through
  * SuperIO. On BMC side the max memory can be mapped is 4kB with the caveat that