Make it clear that the "domain" parameter of the cpg_mstp_attach_dev()
and cpg_mstp_detach_dev() functions is not used.

The cpg_mstp_attach_dev() and cpg_mstp_detach_dev() callbacks are not
only used by the CPG/MSTP Clock Domain driver, but also by the R-Mobile
SYSC PM Domain driver.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>