mtd: nand: Move Toshiba specific init/detection logic in nand_toshiba.c

Move Toshiba specific initialization and detection logic into
nand_toshiba.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>