cp2112: driver for the namesake GPIO and I2C master gadget

Documentation:
- CP2112 Datasheet
  https://www.silabs.com/documents/public/data-sheets/cp2112-datasheet.pdf
- AN495: CP2112 Interface Specification
  https://www.silabs.com/documents/public/application-notes/an495-cp2112-interface-specification.pdf
- CP2112 Errata
  https://www.silabs.com/documents/public/errata/cp2112-errata.pdf

The logic is implemented as three sub-drivers.
The parent driver claims the USB device and creates two child devices.
One acts as a GPIO controller and the other is an I2C controller.

Tested with CP2112 revision F02.
Both features seem to work.
HTU21 sensor was used as an I2C slave.

Reviewed by:	adrian, hselasky
MFC after:	2 weeks
Relnotes:	maybe
Differential Revision: https://reviews.freebsd.org/D25359

sys/conf/files

@@ -3331,6 +3331,7 @@ dev/usb/serial/usb_serial.c 	optional ucom | u3g | uark | ubsa | ubser | \
 #
 # USB misc drivers
 #
+dev/usb/misc/cp2112.c		optional cp2112
 dev/usb/misc/ufm.c		optional ufm
 dev/usb/misc/udbp.c		optional udbp
 dev/usb/misc/ugold.c		optional ugold