Blackfin: bfin_mac: respect CONFIG_PHY_{ADDR,CLOCK_FREQ}

Rather than having the on-chip MAC hardcoded to phy address 1 and a speed
of 2.5mhz, use these as defaults if the board doesn't specify otherwise.

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