boards: antmicro: add support for the Myra SiP Baseboard

Add support for the Antmicro's Myra SiP Baseboard. The board uses
Antmicro's Myra SiP which integrates STM32G491XX MCU and its SoC
configuration.

Signed-off-by: Jakub Wasilewski <jwasilewski@internships.antmicro.com>
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>