stm32mp: stm32prog: correctly handle DM_PMIC

Correctly handle number of alternate when DM_PMIC is not activated.
This patch remove the last UNKNOWN partition in this case.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>