nand: Add function to unregister NAND devices

This performs the opposite of nand_register, allowing drivers to unregister
nand devices. This is probably unnecessary for most regular drivers, but we
expect sandbox drivers to get repeatedly bound/unbound, so this will help
avoid dangling pointers.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>

include/nand.h

@@ -22,6 +22,7 @@ int nand_mtd_to_devnum(struct mtd_info *mtd);
 #if CONFIG_IS_ENABLED(SYS_NAND_SELF_INIT)
 void board_nand_init(void);
 int nand_register(int devnum, struct mtd_info *mtd);
+void nand_unregister(struct mtd_info *mtd);
 #else
 struct nand_chip;