ARM: dts: rockchip: clean up gpio-keys nodes

Drop superfluous #address-cells and #size-cells, rename
key-nodes to individual names and also use the key constants
intead of numbers.

Reported-by: Julien Chauveau <chauveau.julien@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>