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

Fix dtschema validator warnings like:
    l2-cache-controller@48242000: $nodename:0: 'l2-cache-controller@48242000'
        does not match '^(cache-controller|cpu)(@[0-9a-f,]+)*$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>