ARM: dts: stm32: Rename mmc controller nodes to mmc@

Per mmc-controller.yaml, the node pattern is "^mmc(@.*)?$" ,
so adjust the node.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Ludovic Barre <ludovic.barre@st.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: devicetree@vger.kernel.org
Acked-by: Yann Gautier <yann.gautier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>