usb: usb251xb: Update usb251xb bindings

Since hub usb2517 is going to be supported by the usb251xb driver,
the bindings need to be properly updated. Particularly:
- add "microchip,usb2517" and "microchip,usb2517i" compatible strings.
- add "boolean" description to all the properties, which really accept
a boolean value including a new one "led-{usb,speed}-mode".
- move reset-gpios property to the optional section. It isn't
defined as required in the code and shouldn't be required at all, since
hardware may handle reset pins by itself.
- add new led-{usb,speed}-mode mode property. USB2517 device supports
two LED modes: USB mode and speed (default) indication mode. The last one
can be switched on by this property.
- add {bp,sp}-max-{total,removable}-current-microamp property measured in
microamp. It hasn't been defined as property before. Since the limitation
specified by these parameters is hardware specific it needs to be defined
in dts.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>