feat(stm32mp1): add plat_report_*_abort functions

The new helpers are created in STM32MP1 platform for prefetch and data
aborts.
While at it, put plat_report_exception() under DEBUG flag. If DEBUG is
not set, the weak function which does the same will be used.
This plat_report_exception() function can also be simplified, as it will
no more be used to report aborts.

Change-Id: Ibe989b28e236693f317cffb0545ea0611b7bdde4
Signed-off-by: Yann Gautier <yann.gautier@st.com>