dt-bindings: net: dsa: Add compatibility strings for Broadcom Omega

Add compatibility strings for the internal switch in the Broadcom
Omega SoC family (BCM5831X/BCM1140X) to B53.

Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>