drm/xe: Add missing documentation of rpa_freq

drm/xe: Add missing documentation of rpa_freq

While at it, already adjust the rpe_freq frequency, to highlight
that both are calculated by PCODE at runtime.

Fixes: c6aac2fa77a3 ("drm/xe: Introduce the RPa information")
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://lore.kernel.org/r/20250521165146.39616-4-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>