clk: renesas: r9a07g043: Fix HP clock source for RZ/Five

According to the Rev.1.20 hardware manual for the RZ/Five SoC, the clock
source for HP is derived from PLL6 divided by 2.  Correct the
implementation by configuring HP as a fixed clock source instead of a
MUX.

The `CPG_PL6_ETH_SSEL' register, which is available on the RZ/G2UL SoC,
is not present on the RZ/Five SoC, necessitating this change.

Fixes: 95d48d27 ("clk: renesas: r9a07g043: Add support for RZ/Five SoC")
Cc: stable@vger.kernel.org
Reported-by: Hien Huynh <hien.huynh.px@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250127173159.34572-1-prabhakar.mahadev-lad.rj@bp.renesas.com


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