drm/xe/guc: Rename GEN11_SOFT_SCRATCH for clarity

That register is a completely different register, it's not the same as
SOFT_SCRATCH for GEN11 and beyond. Rename to to the same name as the
bspec uses, including the new variant for media. Also, move the
definitions to the guc header.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>