phy: cadence-torrent: Add wrapper for DPTX register access

Add wrapper functions to read, write DisplayPort specific PHY registers to
improve code readability.

Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Signed-off-by: Yuti Amonkar <yamonkar@cadence.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>