remove nand_controller->register_callbacks

Replace flash_driver callback with pointer to command_registration.
Eliminates all related routines and allows drivers to omit commands.

src/flash/s3c2412_nand.c

@@ -64,7 +64,6 @@ static int s3c2412_init(struct nand_device *nand)
 struct nand_flash_controller s3c2412_nand_controller = {
 		.name = "s3c2412",
 		.nand_device_command = &s3c2412_nand_device_command,
-		.register_commands = &s3c24xx_register_commands,
 		.init = &s3c2412_init,
 		.reset = &s3c24xx_reset,
 		.command = &s3c24xx_command,