pinctrl: pinconf-generic: Fully validate 'pinmux' property

In the Linux kernel, the following vulnerability has been resolved: pinctrl: pinconf-generic: Fully validate 'pinmux' property The pinconf_generic_parse_dt_pinmux() assumes that the 'pinmux' property is not empty when present. This might be not true. With that, the allocator will give a special value in return and not NULL which lead to the crash when trying to access that (invalid) memory. Fix that by fully validating 'pinmux' value, including its length.