Blackfin: bf51x: fix alternative portmux options

The BF51x's alternative portmux Kconfig options were copy & pasted from
the BF52x, but never tweaked to reflect it.  So drop the old options as
they were never used (and were simply wrong), and add the BF51x specific
pieces to the Kconfig and header.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>