board: st: common: cleanup dfu support

split the file stm32mp_dfu.c in two files to simplify the Makefile
- stm32mp_dfu.c: required by CONFIG_SET_DFU_ALT_INFO
- stm32mp_dfu_virt.c: required by CONFIG_DFU_VIRT for stm32prog
  command or VIRT device for PMIC for CONFIG_SET_DFU_ALT_INFO.

This patch also remove some remaining #ifdef CONFIG
and avoid compilation error when CONFIG_SET_DFU_ALT_INFO is not
activated.

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