scsi: ufs: core: Introduce a new ufshcd vops negotiate_pwr_mode()

scsi: ufs: core: Introduce a new ufshcd vops negotiate_pwr_mode()

Most vendor specific implemenations of vops pwr_change_notify(PRE_CHANGE)
are fulfilling two things at once:

 - Vendor specific target power mode negotiation

 - Vendor specific power mode change preparation

When TX Equalization is added into consideration, before power mode change
to a target power mode, TX Equalization Training (EQTR) needs be done for
that target power mode. In addition, UFSHCI spec requires to start TX EQTR
from HS-G1 (the most reliable High Speed Gear).

Adding TX EQTR before pwr_change_notify(PRE_CHANGE) is not applicable
because we don't know the negotiated power mode yet.

Adding TX EQTR post pwr_change_notify(PRE_CHANGE) is inappropriate because
pwr_change_notify(PRE_CHANGE) has finished preparation for a power mode
change to negotiated power mode, yet we are changing power mode to HS-G1
for TX EQTR.

Add a new vops negotiate_pwr_mode() so that vendor specific power mode
negotiation can be fulfilled in its vendor specific implementations.  Later
on, TX EQTR can be added post vops negotiate_pwr_mode() and before vops
pwr_change_notify(PRE_CHANGE).

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/20260325152154.1604082-2-can.guo@oss.qualcomm.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>