scsi: ufs: core: Add support to retrain TX Equalization via debugfs

scsi: ufs: core: Add support to retrain TX Equalization via debugfs

Drastic environmental changes, such as significant temperature shifts, can
impact link signal integrity. In such cases, retraining TX Equalization is
necessary to compensate for these environmental changes.

Add a debugfs entry, 'tx_eq_ctrl', to allow userspace to manually trigger
the TX Equalization training (EQTR) procedure and apply the identified
optimal settings on the fly. These entries are created on a per-gear basis
for High Speed Gear 4 (HS-G4) and above, as TX EQTR is not supported for
lower gears.

The 'tx_eq_ctrl' entry currently accepts the 'retrain' command to initiate
the procedure. The interface is designed to be scalable to support
additional commands in the future.

Reading the 'tx_eq_ctrl' entry provides a usage hint to the user, ensuring
the interface is self-documenting.

The ufshcd's debugfs folder structure will look like below:

  /sys/kernel/debug/ufshcd/*ufs*/
  |--tx_eq_hs_gear1/
  |  |--device_tx_eq_params
  |  |--host_tx_eq_params
  |--tx_eq_hs_gear2/
  |--tx_eq_hs_gear3/
  |--tx_eq_hs_gear4/
  |--tx_eq_hs_gear5/
  |--tx_eq_hs_gear6/
     |--device_tx_eq_params
     |--device_tx_eqtr_record
     |--host_tx_eq_params
     |--host_tx_eqtr_record
     |--tx_eq_ctrl

Reviewed-by: Bean Huo <beanhuo@micron.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Can Guo <can.guo@oss.qualcomm.com>
Reviewed-by: Peter Wang <peter.wang@mediatek.com>
Link: https://patch.msgid.link/20260325152154.1604082-8-can.guo@oss.qualcomm.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>