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

Add a wrapper function to write Torrent 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>