ARM: configs: stm32: Add DMAMUX support in STM32 defconfig

This patch adds DMAMUX support in STM32 defconfig file

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>