arm64: dts: marvell: armada-3720: align lednode names with dtschema

The node names should be generic and DT schema expects certain pattern
with 'led'.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220616005333.18491-12-krzysztof.kozlowski@linaro.org