phy: omap-usb2: Clean up exported header

Move private definitions from header to phy-omap-usb2.c file.
Get rid of unused data structures usb_dpll_params and omap_usb_phy_type.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Link: https://lore.kernel.org/r/20200515080518.26870-2-rogerq@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>