Follow the standard nodename pattern
"^(cache-controller|cpu)(@[0-9a-f,]+)*$" defined in
schemas/cache-controller.yaml of dt-schema.

Otherwise, after the dt-binding is converted to json-schema,
'make ARCH=arm dtbs_check' will show warnings like this:

  l2-cache@500c0000: $nodename:0: 'l2-cache@500c0000' does not match '^(cache-controller|cpu)(@[0-9a-f,]+)*$'
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>