phy: renesas: r8a779f0-ether-serdes: add USXGMII mode

The initial driver implementation was limited to SGMII and 1GBit/s. The
new mode allows speeds up to 2.5GBit/s on R-Car S4-8 SOCs.

Signed-off-by: Michael Dege <michael.dege@renesas.com>
Link: https://lore.kernel.org/r/20250703-renesas-serdes-update-v4-1-1db5629cac2b@renesas.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>