dt-bindings: mfd: Add retu/tahvo ASIC chips bindings

There are Device Tree source files defining a device node for the
retu/tahvo I2C chip, but there isn't a DT binding document for it.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>