mtd: nand: Move Micron specific init logic in nand_micron.c

Upstream linux commit 10d4e75c36f6c1.

Move Micron specific initialization logic into nand_micron.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>