drm/xe/uc: Prepare for parsing of different header types

GSC binaries and newer HuC ones use GSC-style headers instead of the
CSS. In preparation for adding support for such parsing, split out the
current parsing code to its own function, to make it cleaner to add the
new paths. The existing doc section has also been renamed to narrow it
to CSS-based binaries.

v2: new patch in series, split out from next patch for easier reviewing
v3: drop unneeded include (Lucas)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>