usb: gadget: Allow up to 900mA current draw for SuperSpeed

Currently 500mA is used as max allowable current to draw over
USB VBUS. However in SuperSpeed a gadget may draw up to 900mA
Update the CONFIG_USB_GADGET_VBUS_DRAW's allowable range to
be up to 900. But depending on connection speed, ensure that
high speed and slower are capped to 500mA when encoding the
bMaxPower value in the configuration descriptor.

Change-Id: Iae9ecf586135b0a2064e7d5e6e8fa3d8e7e4fb70
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>