Blackfin: BF518F-EZBRD: handle required portmuxing of async pins

The two high address lines on the BF51x are not dedicated which means we
need to handle them like any other peripheral pin if we want to access the
upper 2MB of parallel flash.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>