phy: phy-omap-pipe3: Add support for PCIe PHY

PCIe PHY uses an external pll instead of the internal pll used by SATA
and USB3. So added support in pipe3 PHY to use external pll.

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