pinctrl: imx: Do not treat a PIN without MUX register as an error

Some PINs do not have a MUX register, it is not an error.
It is necessary to allow the continuation of the PINs configuration,
otherwise the whole PIN-group will be configured incorrectly.

Cc: stable@vger.kernel.org
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>