MIPS: BCM47xx: Extract info about et2 interface

MIPS: BCM47xx: Extract info about et2 interface

New devices may have more than 1 Ethernet core (device). We should
extract info about them to make it available to Ethernet drivers.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Hante Meuleman <meuleman@broadcom.com>
Cc: Ian Kent <raven@themaw.net>
Patchwork: https://patchwork.linux-mips.org/patch/10027/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>