nand: Add common functions to linux/mtd/nand.h

Functions often used in SPL are now part of linux/mtd/nand.h.
Static modifiers are removed from these functions in
drivers/mtd/nand/nand_base.c.

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
Cc: scottwood@freescale.com
Cc: s-paulraj@ti.com
Cc: albert.u.boot@aribaud.net
Acked-by: Scott Wood <scottwood@freescale.com>