mtd: rawnand: Replace of_gpio_named_count() by gpiod_count()

As a preparation to unexport of_gpio_named_count(), convert the
driver to use gpiod_count() instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220830183336.49966-1-andriy.shevchenko@linux.intel.com