gpio: gpio-mm-lantiq: Use of_property_read_u32

Instead of parsing manually the shadow content, use the much simpler
helper of_property_read_u32.

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: John Crispin <blogic@openwrt.org>
Cc: devicetree@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>