dt-bindings: mfd: tps6594: Allow gpio-line-names

Setting the signal names in the device tree was already possible, but
it will lead to a warning. Allow the gpio-line-names property to fix
that.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Acked-by: "Rob Herring (Arm)" <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>