Blackfin: bfin_mac: convert to portmux framework

Rather than bang MMRs directly, use the new portmux framework to handle
the details.  While we're doing this, let boards declare the exact list
of pins they need in case there is one or two they don't actually have
hooked up.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>