ARM: multi_v7_defconfig: enable STM32 dfsdm audio support

Add STM32 DFSDM audio support by enabling CONFIG_SND_SOC_STM32_DFSDM
as module.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>