feat(stm32mp13-fdts): use STM32MP_DDR_S_SIZE in fw-config

Align with STM32MP15 file, use the macro STM32MP_DDR_S_SIZE, instead of
an hard-coded value.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ib31bed1ffe89ff221fab1884a2db729ce1e21846
1 file changed