mtd: nand: Move Hynix specific init/detection logic in nand_hynix.c

Move Hynix specific initialization and detection logic into
nand_hynix.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>