ARM: dts: BCM5301X: Add a default compatible for switch node

Provide a default compatible string which is based on the 53011 SRAB
compatible by default. The 4709 and 47094 default to the 53012 SRAB
compatible.

This allows us to have sane defaults and silences the following
warnings:

arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dt.yaml:
ethernet-switch@18007000: compatible: 'oneOf' conditional failed, one
must be fixed:
        ['brcm,bcm5301x-srab'] is too short
        'brcm,bcm5325' was expected
        'brcm,bcm53115' was expected
        'brcm,bcm53125' was expected
        'brcm,bcm53128' was expected
        'brcm,bcm5365' was expected
        'brcm,bcm5395' was expected
        'brcm,bcm5389' was expected
        'brcm,bcm5397' was expected
        'brcm,bcm5398' was expected
        'brcm,bcm11360-srab' was expected
        'brcm,bcm5301x-srab' is not one of ['brcm,bcm53010-srab',
'brcm,bcm53011-srab', 'brcm,bcm53012-srab', 'brcm,bcm53018-srab',
'brcm,bcm53019-srab']
        'brcm,bcm5301x-srab' is not one of ['brcm,bcm11404-srab',
'brcm,bcm11407-srab', 'brcm,bcm11409-srab', 'brcm,bcm58310-srab',
'brcm,bcm58311-srab', 'brcm,bcm58313-srab']
        'brcm,bcm5301x-srab' is not one of ['brcm,bcm58522-srab',
'brcm,bcm58523-srab', 'brcm,bcm58525-srab', 'brcm,bcm58622-srab',
'brcm,bcm58623-srab', 'brcm,bcm58625-srab', 'brcm,bcm88312-srab']
        'brcm,bcm5301x-srab' is not one of ['brcm,bcm3384-switch',
'brcm,bcm6328-switch', 'brcm,bcm6368-switch']
        From schema:
Documentation/devicetree/bindings/net/dsa/b53.yaml

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