net: dsa: b53: Add compatible strings for the Cygnus-family BCM11360.

Cygnus is a small family of SoCs, of which we currently have
devicetree for BCM11360 and BCM58300.  The 11360's B53 is mostly the
same as 58xx, just requiring a tiny bit of setup that was previously
missing.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>