ARM: dts: socfpga: Align L2 cache-controller nodename with dtschema

Fix dtschema validator warnings like:
    l2-cache@fffff000: $nodename:0:
        'l2-cache@fffff000' does not match '^(cache-controller|cpu)(@[0-9a-f,]+)*$'

Fixes: 475dc86d ("arm: dts: socfpga: Add a base DTSI for Altera's Arria10 SOC")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>