mtd: nand: drop edb7312 support

EDB7312 isn't supported by mainline kernel, so drop it now.
If the board support will ever be submitted to mainline,
one can revert this commit.

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