Add @platform doxygen alias

The @platform alias will describe which platform (for example,
gpu_baremetal or/and host) an API can be used.

The get_platform.py is a tool to compare APIs in different platforms.

Change-Id: I902bc4fea048269eace6e9f3f4a8e93f3adb7f87

docs/doxygen/Doxyfile

@@ -2681,4 +2681,4 @@ GENERATE_LEGEND        = YES
 
 DOT_CLEANUP            = YES
 
-ALIASES += usecase{1}="\xrefitem usecase \"Use cases\" \"Use cases\" \1"
+ALIASES += platform{1}="\xrefitem platform \"Platform\" \"Platforms\" \1"