doc: DTS: Update DWC3 binding to provide reference to generic bindings

Since the Synopsys DWC3 controller driver inherits the generic bindings
defined in 'usb/generic.txt', this patch tries to capture the same in
the DWC3 binging documentation to avoid any confusion in usage of properties
like 'dr_mode' for certain SoCs like FSL LS2080A.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>