mtd: rawnand: lpc32xx_slc: Convert to use devm_gpiod_get_optional()

mtd: rawnand: lpc32xx_slc: Convert to use devm_gpiod_get_optional()

The initial fix for a GPIO descriptor leak added manual gpiod_put()
calls in the error path and remove function.

This follow-up patch improves upon the fix by switching to the
resource-managed devm_gpiod_get_optional() API.

Suggested-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>