mtd: rawnand: atmel: clarify NAND addition/removal paths

No need for an atmel_nand_register() function, let's move the code in
it directly where the function was called: in
atmel_nand_controller_add_nand(). To make things consistent, also
rename atmel_nand_unregister() into
atmel_nand_controller_remove_nand().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>