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.hpp b/lpc_nuvoton.hpp
index bf53e29..b72a673 100644
--- a/lpc_nuvoton.hpp
+++ b/lpc_nuvoton.hpp
@@ -29,6 +29,8 @@
         regionAddress(regionAddress),
         sys(sys){};
 
+    void close() override;
+
     std::pair<std::uint32_t, std::uint32_t>
         mapWindow(std::uint32_t address, std::uint32_t length) override;