ARM: dts: stm32: Set spi-rx/tx-bus-width to 4 for stm32f746-eval

As n25q512a qspi flash supports quad input fast program and
quad input fast read, set spi-tx_bus-width and spi-rx_bus-width
to 4.

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