[MTD] Don't include linux/mtd/map.h from linux/mtd/physmap.h

Replace the inclusion of linux/mtd/map.h with a forward-declaration
of struct map_info. This allows linux/mtd/physmap.h to be included by
e.g. board code even if the MTD subsystem is disabled.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>