fix(st-ddr): add missing debug.h

In a later patch, the stm32mp1_def.h will be reworked. The inclusion
of common/debug.h may not be done there through another included file.
Add this header inclusion in the files that need it.

Change-Id: I83687f7910032ca38c0856796580a650e1e41a68
Signed-off-by: Yann Gautier <yann.gautier@st.com>