dt-bindings: mfd: Add TI tps6105x chip bindings

There are Device Tree source files defining a device node for the
tps61050/61052 I2C chip but there isn't a binding document for it.

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