usb: cdns3: core: get role switch node from firmware

After that, the role switch device (eg, Type-C device) could call
cdns3_role_set to finish the role switch.

Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>