boards: stm32l5: Refactor serial peripheral descriptions

On stm32l5 based boards, some serial devices descriptions were
scattered between common.dtsi and base or non secure dts files.

This leads to build issues now that pinctrl-0 and pinctrl-names
properties are required but this was anyway broken since the
beginning.

Group all serial devices definition in common.dtsi so complete devices
description is available to both non secure and base descriptions.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>