ARM: dts: armada-385-linksys: label nodes

Add labels to nodes used by dependants. Also rename node gpio_keys to
gpio-keys to match the style of the rest of the file as well as the
documented example.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>