ARM: dts: BCM5301X: Update Ethernet switch node name

Update the switch unit name from srab to ethernet-switch, allowing us to
fix warnings such as:

  CHECK   arch/arm/boot/dts/bcm4708-buffalo-wzr-1750dhp.dt.yaml
arch/arm/boot/dts/bcm4708-buffalo-wzr-1750dhp.dt.yaml:
srab@18007000: $nodename:0: 'srab@18007000' does not match
'^(ethernet-)?switch(@.*)?$'
        From schema:
Documentation/devicetree/bindings/net/dsa/b53.yaml

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>