usb: dwc3: add support for USB 2.0-only core configuration

Newer DWC3 controllers can be built for USB 2.0-only mode, where
most of the USB 3.0 circuitry is left out. To support this mode,
the driver must limit the speed programmed into the DCFG register
to Hi-Speed or lower.

Reads and writes to the PIPECTL register are left as-is, since
they should be no-ops in USB 2.0-only mode. Calls to phy_init()
etc. for the USB3 phy are also left as-is, since the no-op USB3
phy should be used for USB 2.0-only mode controllers.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>