usb: dwc3: dwc3-omap: change probe and remove to uboot init and uboot exit code

Removed probe and remove that are specific to linux and replaced it with
uboot init and uboot exit. These functions will be invoked from boardfile.

This will change once we have dwc3-omap driver adapted to use the uboot
driver model.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>