[MTD] add get_mtd_device_nm() function

This patch adds one more function to the MTD interface to make it possible to
open MTD devices by their names, not only numbers. This is very handy in many
situations. Also, MTD device number depend on load order and may vary, while
names are fixed.

Signed-off-by: Artem Bityutskiy <dedekind@infradead.org>