Add Tahvo USB transceiver driver.

Based on old code from linux-omap tree. The original driver was written
by Juha Yrjölä, Tony Lindgren, and Timo Teräs.
Signed-off-by: NAaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: NFelipe Balbi <balbi@ti.com>