mtd: nand: Export nand_reset() symbol

Commit 6e532afa ("mtd: nand: atmel: Add PM ops") started to use the
nand_reset() function which was not yet exported by the NAND framework
(because it was only used internally before that). Export this symbol
to avoid build errors when the driver is enabled as a module.

Fixes: 6e532afa ("mtd: nand: atmel: Add PM ops")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>