samples: boards: stm32: Add a serial wakeup sample

Add a sample to demonstrate use of (LP)U(S)ART with wakeup capability
This sample is provided with 2 configurations
- nucleo_wb55rg: Basic configuration that could be exercised in CI
- stm32l562e_dk: "Full feature" configuration that require some set up
configuration (be able to display console from lpuart) and allows to
achieve lowest power consumption.

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