clk: renesas: r8a7791/r8a7793: Fix LB clock divider

The CLK_TYPE_GEN2_LB clock type is meant for SoCs like R-Car H2, where
the LB clock divider depends on the value of the MD18 pin.

On R-Car M2-W and M2-N, the LB clock divider is fixed to 24.  Hence
model the clock as a fixed factor clock instead.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>