usb: common: add support to get maximum speed from dt

Add support to get maximum speed from dt so that usb drivers
makes use of it for DT parsing.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
(rebase and fix errors)
Reviewed-by: Simon Glass <sjg@chromium.org>