pinctrl: renesas: rzg2l: Parameterize OEN register offset

pinctrl: renesas: rzg2l: Parameterize OEN register offset

Prepare for supporting SoCs with varying OEN register locations by
parameterizing the OEN offset in the rzg2l driver. Introduce an `oen`
field in the rzg2l_register_offsets structure and update rzg2l_read_oen(),
rzg2l_write_oen(), suspend/resume caching, and SoC hwcfg entries to use
this offset instead of the hard-coded ETH_MODE value.

As part of this change, rename the field `eth_mode` in the register cache
to `oen` to better reflect its general purpose and decouple the naming
from a specific register.

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/20250806195555.1372317-3-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>