arm: stm32mp1: Set soc_type, soc_pkg, soc_rev env variables

Split up get_soc_name(), clean the decoding up a bit, and set up
environment variables which contain the SoC type, package, revision.
This is useful on SoMs, where multiple SoC options are populated.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>