ARM: dts: BCM5301X: update CRU block description

This describes CRU in a way matching documentation and fixes:

arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dt.yaml: cru@100: $nodename:0: 'cru@100' does not match '^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'
        From schema: /lib/python3.6/site-packages/dtschema/schemas/simple-bus.yaml

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>