mtd: nand: Move AMD/Spansion specific init/detection logic in nand_amd.c

Move AMD/Spansion specific initialization/detection logic into
nand_amd.c. This is part of the "separate vendor specific code from
core" cleanup process.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Richard Weinberger <richard@nod.at>