fix(stm32mp1): add missing debug.h

Due to stm32mp_shres_helpers.h removal, the debug.h header is no more
included. It should then be added to stm32mp1_boot_device.c.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I397911ac05fdff464c010cf3b2e04320a781b4aa