clk: renesas: rzg2l: Rename mstp_clock to mod_clock

The mstp_clock structure really represents a module clock (cfr. the
various rzg2l_mod_clock_*() functions and the to_mod_clock() helper),
and is not directly related to the "Module stop state".
Rename it to "mod_clock", and replace "mstp_clock" by "mod_clock".
to avoid confusion with the mstop registers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://lore.kernel.org/53b3a730a784650762cdb27fdbde7a45b0c20db8.1749119264.git.geert+renesas@glider.be