stm32mp: stm32prog: Remove usage of "mtdparts" function

stm32mp: stm32prog: Remove usage of "mtdparts" function

Motivation for this patch is to remove usage of function define
in cmd/mtdparts.c interface, based on env variables mtdids and mtdparts:
mtdparts_init() and find_dev_and_part().
See commit 53dac68014d7 ("cmd: mtdparts: describe as legacy")

Now, all MTD devices are populated with their partition's information
found in DT, accessible in MTD devices. Use these information to find
the wanted partitions, no more need of find_dev_and_part() usage.

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