clk: renesas: rzg2l: Add FOUTPOSTDIV clk support

PLL5 generates FOUTPOSTDIV clk and is sourced by LCDC/DSI modules.
The FOUTPOSTDIV is connected to PLL5_4 MUX. Video clock is sourced
from DSI divider which is connected to PLL5_4 MUX.

This patch adds support for generating FOUTPOSTDIV clk.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20220430114156.6260-2-biju.das.jz@bp.renesas.com


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