scsi: ufs: dt-bindings: Document static TX Equalization settings properties

UFS v5.0/UFSHCI v5.0 adds HS-G6 support (46.6 Gbps/lane) via UniPro v3.0
and M-PHY v6.0. These specs define TX Equalization for all High-Speed Gears
(not only HS-G6) to compensate channel loss and improve signal integrity at
high speed.

For HS-G6, M-PHY uses PAM4 1b1b line coding. Pre-Coding may also be
required depending on channel characteristics.

Document vendor-neutral properties in ufs-common.yaml:

 - txeq-preshoot-g[1-6]

 - txeq-deemphasis-g[1-6]

 - tx-precode-enable-g6

Values are per-lane Host/Device tuples (2 values for x1, 4 values for
x2). PreShoot/DeEmphasis range from 0..7, and Precode is 0/1.

These are board-specific signal-integrity tuning values. They depend on
channel SI/PHY characterization and validation (host PHY, device PHY,
package, and board routing), and are determined by HW/PHY designers.

Although UFSHCI v5.0 supports TX Equalization Training via UniPro v3.0,
which allows host software to determine optimal TX Equalization at
runtime, static board-specific TX Equalization settings in the Device

Tree are still necessary because:

 - TX Equalization Training is not supported for HS-G3 and below

 - TX Equalization Training is disabled on some platforms

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Peter Wang <peter.wang@mediatek.com>
Signed-off-by: Can Guo <can.guo@oss.qualcomm.com>
Link: https://patch.msgid.link/20260616113348.1168248-2-can.guo@oss.qualcomm.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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