stm32mp: stm32prog: Add support of FWU_MDATA partition type

Add support of "FWU_MDATA" partition type in flashlayout to select
the TF-A firmware update metadata partition type guid, associated to
U-Boot "system" partition type guid, FWU_MDATA_GUID introduced by
commit 2eaedc95 ("FWU: Add FWU metadata structure and driver for
accessing metadata") and used in gpt_get_mdata_partitions() for
commit 554b38f7 ("FWU: Add FWU metadata access driver for GPT
partitioned block devices")

See also recommendation in FWU-PSA-A_DEN0118_1.0ALP3.pdf
  4.1.2 Metadata integration with GPT
  When embedded in a GPT, each metadata replica occupies a single
  partition with PartitionTypeGUID = metadata_uuid.
  UUID = 8a7a84a0-8387-40f6-ab41-a8b9a5a60d23

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