dt-bindings: gpio: Add gpio-line-names to STMPE GPIO

This is a gpio-controller, so gpio-line-names should be allowed as well.
stmpe2403 supports up to 24 GPIOs.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>