mtd: uclinux: add a comment about why uclinux_ram_map must not be static

I was (at least) the second person trying to fix a warning by sparse, so
document in the code why this is a bad idea and add an extern declaration to
make sparse happy.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>