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

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