phy: marvell: phy-mvebu-cp110-comphy: rename instances of DLT

The documentation for Marvell's cp110 phy refers to these
registers/register regions as DTL control, DTL frequency loop enable,
etc. This patch aligns the relevant code for these accordingly.

Signed-off-by: Matt Pelland <mpelland@starry.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>