feat(stm32mp2): add STM32MP_USB_PROGRAMMER compilation

Add minimal compilation step when enabling STM32MP_USB_PROGRAMMER flag
on STM32MP2. Add DWL_BUFFER_BASE in platform.mk and the compilation
of the new file plat/st/stm32mp2/stm32mp2_usb_dfu.c (just stubs for
the moment).

Signed-off-by: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I8891ff23ddc3d40d7477ada3e49e439dd8af8316
2 files changed