phy: cadence-torrent: Use regmap to read and write Torrent PHY registers

Use regmap for accessing Torrent PHY registers. Modify register offsets
as defined in Torrent PHY user guide. Abstract address calculation
using regmap APIs.

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>