MTD: NAND: ams-delta: use GPIO instead of custom I/O

Don't use Amstrad Delta custom I/O functions for controlling the device,
use GPIO API instead.

While being at it, add missing gpio_free(AMS_DELTA_GPIO_PIN_NAND_RB).

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Cc: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>