pinctrl: renesas: rza2: Add null check in the rza2_gpio_register

When devm_kasprintf fails, it returns a null pointer. However, this return value is not properly checked in the function rza2_gpio_register. 

A check should be added after the devm_kasprintf call to prevent potential null pointer dereference issues.

CODE:
	chip.label = devm_kasprintf(priv->dev, GFP_KERNEL, "%pOFn", np);
	chip.parent = priv->dev;
	chip.ngpio = priv->npins;