phy: ti-pipe3: streamline PHY operations

Limit .power_on() and .power_off() to just control the
PHY power and not the DPLL. The DPLL will be enabled
in .init() and idled in .exit().

Don't reprogram the DPLL if it has been already locked
by the bootloader. This fixes a problem with SATA, where
it fails if SATA was used by the bootloader.

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