mtd: maps: drop edb7312 support

EDB7312 isn't supported by mainline kernel, this driver wasn't working
before recent fixes, the same functionality can be achieved via physmap,
so drop it now. If the board support will ever be submitted to mainline,
one either can revert this commit, or use physmap mtd map driver.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>