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

Upstream linux commit 229204da53b31d.

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: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>