pinctrl: apple: return an error if pinmux is missing in the DT

If of_property_count_u32_elems returned 0, return -EINVAL to indicate
a failure. Previously this would return 0.

Fixes: a0f160ff ("pinctrl: add pinctrl/GPIO driver for Apple SoCs")
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>