ARM: dts: mmp3: Fix L2 cache controller node name

The current one makes validation unhappy:

  mmp3-dell-ariel.dt.yaml: l2-cache-controller@d0020000: $nodename:0:
      'l2-cache-controller@d0020000' does not match
      '^(cache-controller|cpu)(@[0-9a-f,]+)*$'

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>