auxdisplay: img-ascii-lcd: Only build on archs that have IOMEM

This avoids the MODPOST error:

  ERROR: "devm_ioremap_resource" [drivers/auxdisplay/img-ascii-lcd.ko] undefined!

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>