dt-bindings: gpio: mpfs: allow gpio-line-names

The BeagleV Fire devicetree will make use of gpio-line-names, allow it
in the binding.

Signed-off-by: Jamie Gibbons <jamie.gibbons@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>