dt-bindings: i2c-stm32: document optional dmas

Add missing documentation for "dmas" and "dma-names" properties that can be
used on i2c-stm32.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>