net: dsa: bcm_sf2: Allow non-IMP ports to have Broadcom tags enabled

Parse the "brcm,use-bcm-hdr" boolean property during ports
identification to fill a bitmask of ports that should have Broadcom tags
enabled. This is needed in some configurations where per-packet metadata
can be exchanged using Broadcom tags between the switch and an on-chip
acceleration device.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>