drm/imagination: Add GPU register headers

Changes since v8:
- Corrected license identifiers

Changes since v5:
- Split up header commit due to size

Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/9f1fbf6c18e9644ac10712e05893701f06aee6ae.1700668843.git.donald.robson@imgtec.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>