Blackfin: bf537-stamp: declare parallel flash as ROM with XIP kernel

When the kernel is executing out of parallel flash (XIP), we can't have
the flash go into an erase/programming cycle, otherwise the instruction
fetching steps fail and everything crashes.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>