dt-bindings: mfd: Fix documentation of tps65911

This patch fixes documentation of tps65911 because its list of
compatible regulators contains wrongly vdd3 instead of vdd2.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>