Correct the following sparse warnings:
plat/st/common/stm32mp_dt.c:103:5: warning:
 symbol 'fdt_get_node_parent_address_cells' was not declared.
 Should it be static?
plat/st/common/stm32mp_dt.c:123:5: warning:
 symbol 'fdt_get_node_parent_size_cells' was not declared.
 Should it be static?

As those 2 functions are only used by assert(), put them under
ENABLE_ASSERTIONS flag.

Change-Id: Iad721f12128df83a3de3f53e7920a9c1dce64c56
Signed-off-by: Yann Gautier <yann.gautier@st.com>