phy: ti-pipe3: Introduce mode property in driver data

Introduce a mode property in the driver data so that
we don't have to keep using "of_device_is_compatible()"
throughtout the driver.

No functional change.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>