drm/i915/guc: Add GuC css header parser

drm/i915/guc: Add GuC css header parser

The size / offset information of all firmware ingredients are
now caculated from header. Driver will validate the header and
rsa key size. If any component is out of boundary, driver will
reject the loading too.

v6: Clean up warnings from make docs

v5: Tidy up GuC titles in kernel/Doc

v4: Now using 'size_dw' for those defined in css_header

v3: 1) Move DOC to intel_guc_fwif.h right before css_header
definition. Add more comments.
    2) Change 'size' to 'len' or 'length' to avoid confusion.
    3) Add UOS_RSA_SCRATCH_MAX_COUNT according to BSpec. And
driver validate size of RSA key now.
    4) Add fw component size/offset info to intel_guc_fw.

v2: Add indent into DOC to make fixed-width format rather than
change the tmpl.

v1: 1) guc_css_header is defined as __packed now
    2) Add and correct GuC related topics in kernel/Doc

Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>