boards: stm32g0316: use pinmux remap dts bindings

PA11 can be remapped to PA9 by using the recently introduced
remap-pa11-as-pa9 property.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>