clk: renesas: rzg2l: Simplify .determine_rate()

rzg2l_cpg_sd_clk_mux_determine_rate() is the same as
__clk_mux_determine_rate_closest(), so use the latter to save some LoC.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/fed02e0325275df84e2d76f8c481e40e7023cbd9.1688760372.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>