board: st: common: Fix board_get_alt_info_mtd()

Since MTD devices are partioned, we got the following
error when command "dfu 0" is executed:

DFU alt info setting: done
ERROR: Too many arguments for nor0
ERROR: DFU entities configuration failed!
ERROR: (partition table does not match dfu_alt_info?)

Fixes: 31325e1b ("stm32mp1: dynamically build DFU_ALT_INFO")

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