arm64: dts: armada-3720: align LED node name with bindings

Bindings expect the LED node names to follow certain pattern, see
dtbs_check warnings:

  armada-3720-gl-mv1000.dtb: leds: 'power', 'vpn', 'wan' do not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>