clk: renesas: rzg2l: Simplify rzg2l_cpg_assert() and rzg2l_cpg_deassert()

clk: renesas: rzg2l: Simplify rzg2l_cpg_assert() and rzg2l_cpg_deassert()

Combine common code from rzg2l_cpg_assert() and rzg2l_cpg_deassert() into a
new __rzg2l_cpg_assert() helper to avoid code duplication. This reduces
maintenance effort and improves code clarity.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://lore.kernel.org/20250903082757.115778-2-tommaso.merciai.xr@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>