mtd: nand: import nand_hw_control_init()

The code to initialize a struct nand_hw_control is duplicated across
several drivers. Factorize it using an inline function.

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>