dt-bindings: memory-controller: add STM32 FMC2 EBI controller documentation

This patch adds the documentation of the device tree bindings for the STM32
FMC2 EBI controller.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1591975362-22009-4-git-send-email-christophe.kerello@st.com