board/argonkey/dts: Configure i2c3 at 400KHz

As explained in issue #8915 the STM32F4xx SoC family I2C
does not work well in Standard mode (100KHz). So let's
configure i2c3 in Fast mode (400KHz).

Signed-off-by: Armando Visconti <armando.visconti@st.com>