clk: renesas: rzg2l: Lock around writes to mux register

The SD MUX output (SD0) is further divided by 4 in G2{L,UL}.  The
divided clock is SD0_DIV4. SD0_DIV4 is registered with
CLK_SET_RATE_PARENT which means a rate request for it is propagated to
the MUX and could reach rzg2l_cpg_sd_clk_mux_set_parent() concurrently
with the users of SD0.
Add proper locking to avoid concurrent accesses on SD MUX set rate
registers.

Fixes: eaff3364

 ("clk: renesas: rzg2l: Add SDHI clk mux support")
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230929053915.1530607-4-claudiu.beznea@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>