mfd: retu: Add Tahvo support

Tahvo is a multi-function device on Nokia 770, implementing USB
transceiver and charge/battery control.

It's so close to Retu that a single driver can support both.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>