[MTD] Provide mtdram.h with mtdram_init_device() prototype

This is used by axisflashmap.c to boot from ram.

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Acked-by: Mikael Starvik <starvik@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>