arm: stm32mp: add function get_soc_name

Add a function get_soc_name to get a string with the full name
of the SOC "STM32MP15xxx Rev.x"

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