feat(fwu): pass a const metadata structure to platform routines

The metadata structure copy is passed to the platform routine to set
the image source to boot the platform from. This is done by reading
the metadata structure. Pass the metadata as a read-only copy to the
routine -- the routine only needs to consume the metadata values and
should not be able to update the metadata fields.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I399cad99ab89c71483e5a32a1de0e22df304f8b0
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 24af13e..be04b97 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -889,7 +889,7 @@
 
 ::
 
-    Argument : struct fwu_metadata *metadata
+    Argument : const struct fwu_metadata *metadata
     Return   : void
 
 This function is mandatory when PSA_FWU_SUPPORT is enabled.