net: stmmac: dwmac-renesas-gbeth: Add support for RZ/T2H SoC

net: stmmac: dwmac-renesas-gbeth: Add support for RZ/T2H SoC

Extend the Renesas GBETH stmmac glue driver to support the RZ/T2H SoC,
where the GMAC is connected through a MIIC PCS. Introduce a new
`has_pcs` flag in `struct renesas_gbeth_of_data` to indicate when PCS
handling is required.

When enabled, the driver parses the `pcs-handle` phandle, creates a PCS
instance with `miic_create()`, and wires it into phylink. Proper cleanup
is done with `miic_destroy()`. New init/exit/select hooks are added to
`plat_stmmacenet_data` for PCS integration.

Update Kconfig to select `PCS_RZN1_MIIC` when building the Renesas GBETH
driver so the PCS support is always available.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/20250908105901.3198975-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>