MIPS: dts: correct gpio-keys names and properties

gpio-keys children do not use unit addresses.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>