ram: stm32mp1: Add support for multiple configs

Add support for multiple DRAM configuration subnodes, while retaining
the support for a single flat DRAM configuration node. This is useful
on systems which can be manufactured in multiple configurations and
where the DRAM configuration can be determined at runtime.

The code is augmented by a function which can be overridden on board
level, allowing a match on the configuration node name, very much like
the fitImage configuration node name matching works. The default match
is on the single top-level DRAM configuration, if matching on subnodes
is required, then this board_stm32mp1_ddr_config_name_match() must be
overridden.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>