ARM: dts: stm32: limit sdio frequency to 14Mhz for stm32f429i-eval

This avoids the following errors while reading on mmc:
  Read data bytes CRC error: 0x2
  switch to partitions #0, OK
  mmc0 is current device
  Read data bytes CRC error: 0x82002
  ** Unrecognized filesystem type **

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>