HID: add hid-cp2112 driver

HID: add hid-cp2112 driver

This patch adds support for the Silicon Labs CP2112 "Single-Chip HID USB to
SMBus Master Bridge."

This is a HID device driver which registers as an i2c adapter and gpiochip to
expose these functions of the CP2112. The customizable USB descriptor fields
are exposed as sysfs attributes.  The SMBus byte-read, byte-data-read/write,
and word-data-read transfer modes have been tested by talking to an i2c
sensor.  The GPIO functions and USB descriptor field programming have also
been tested.

Signed-off-by: David Barksdale <dbarksdale@uplogix.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>