net: ravb: Add OF probing support

Add support for probing the RAVB Ethernet block from device tree.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>