mtd: nand: atmel: mark resume function __maybe_unused

The newly added suspend/resume support causes a harmless warning:

drivers/mtd/nand/atmel/nand-controller.c:2513:12: error: 'atmel_nand_controller_resume' defined but not used [-Werror=unused-function]

This shuts up the warning with a __maybe_unused annotation.

Fixes: b107007a7114 ("mtd: nand: atmel: Add PM ops")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>