i2c: Add bus driver for for OSIF USB i2c device.

OSIF, Open Source InterFace, is a USB based i2c bus master.  The
origional design was based on i2c-tiny-usb, but more modern versions
of the firmware running on the MegaAVR microcontroller use a different
protocol over the USB. This code is based on Barry Carter
<barry.carter@gmail.com> driver.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>