stm32mp1: remove useless LIBFDT_SRCS from PLAT_BL_COMMON_SOURCES

Remove inclusion of libfdt source files in source file list since these
are already included from generic libfdt.mk makefile.

Change-Id: If42624557c4d88ca85d70f83b1b08c58f50afe72
Signed-off-by: Yann Gautier <yann.gautier@st.com>