usb: common: Parse for USB SSP genXxY

The USB "maximum-speed" property can now take the SSP signaling rate
generation and lane count with these new strings:

"super-speed-plus-gen2x2"
"super-speed-plus-gen2x1"
"super-speed-plus-gen1x2"

Introduce usb_get_maximum_ssp_rate() to parse for the corresponding
usb_ssp_rate enum. The original usb_get_maximum_speed() will return
USB_SPEED_SUPER_PLUS if it matches one of these new strings.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/f8ed896313d8cd8e2d2b540fc82db92b3ddf8a47.1611106162.git.Thinh.Nguyen@synopsys.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>