net: ethernet: ti: am65-cpsw: fix up PHY mode for fixed RGMII TX delay

The am65-cpsw driver currently sets the SEL_RGMII_IDMODE flag in a MAC's
mode register to enable or disable the TX delay. While this was supported
for earlier generations of the CPSW controller, the datasheets of all
modern TI SoCs using the am65-cpsw MAC state that the TX delay is fixed,
and the SEL_RGMII_IDMODE bit is documented as reserved in most of them.
Furthermore, while it was found that this bit does in fact disable the TX
delay, [1] states that this setting is truly unsupported by TI and not
just undocumented.

Following the clarification of the rgmii* phy-mode values in the Linux
DT bindings in [2], the Linux am65-cpsw driver was changed in [3] to
account for the fixed TX delay by fixing up the mode passed to the PHY
driver; a similar fixup already existed in the TI icssg-prueth driver.
[4] followed up on this by explicitly clearing the SEL_RGMII_IDMODE flag
to handle the case where it is set by the bootloader or other firmware
before Linux.

With the above changes, Device Trees that set the recommended "rgmii-id"
mode are now appearing in Linux 6.17+. Avoid setting the unsupported
SEL_RGMII_IDMODE flag for such Device Trees, and instead fix up the PHY
interface mode, thus aligning the U-Boot driver with the Linux kernel.

[1] https://www.spinics.net/lists/netdev/msg1112647.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c360eb0c3ccb95306704fd221442283ee82f1f58
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ca13b249f291f4920466638d1adbfb3f9c8db6e9
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a22d3b0d49d411e64ed07e30c2095035ecb30ed2

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>

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