Ethernet reference clock can be sourced from PLL5_FOUT3 or PLL6. Add
support for ethernet source clock selection using SEL_PLL_6_2 mux.

This patch also renames the PLL5_DIV2 core clock to PLL5_250 to match
with the register description as mentioned in RZ/G2L HW manual (Rev.1.00).
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20210922155145.28156-3-biju.das.jz@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>