dt-bindings: phy: mediatek,mt7988-xfi-tphy: add new bindings

Add bindings for the MediaTek XFI Ethernet SerDes T-PHY found in the
MediaTek MT7988 SoC which can operate at various interfaces modes:

via USXGMII PCS:
 * USXGMII
 * 10GBase-R
 * 5GBase-R

via LynxI SGMII PCS:
 * 2500Base-X
 * 1000Base-X
 * Cisco SGMII (MAC side)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/da5498096f71a40ca1eac4124b7bb601c82396fb.1712625857.git.daniel@makrotopia.org


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