usb: make usb descriptor power options configurable

Add two new kconfig options USB_SELF_POWERED and USB_MAX_POWER.
These can be set by the user to change the USB configuration descriptor.
USB_MAX_POWER can be set to any value between 0 and 250, but practically
should be 50 or 250. These values are half the ammount of mA that the
device will tell the host that it needs.
USB_SELF_POWERED sets the 7th bit in bmAttributes of the USB config
descriptor. Should be set to y if the device has its own power source
other than USB.

Signed-off-by: Barry Solomon <barry.solomon@dexcom.com>