stm32mp: stm32prog: use get_cpu_dev for GetID command

Use get_cpu_dev() in uart getID command and remove the defines
DEVICE_ID_BYTE1 and 2 defines.

This patch prepare the support for new SOC family.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>