ARM: dts: kirkwood: Fix interrupt controller node name

The current ones makes validation unhappy:

  kirkwood-lsxhl.dt.yaml: main-interrupt-ctrl@20200: $nodename:0:
    'main-interrupt-ctrl@20200' does not match
    '^interrupt-controller(@[0-9a-f,]+)*$'

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