configs: stm32f746-disco: Enable SPI_FLASH_MACRONIX

This config file is shared between stm32f746-disco and stm32f769-disco.
These 2 boards doesn't embed the same qspi flash (Micron n25q128a on
f746-disco, Macronix mx66l51235l on f769-disco).

To be able to use Macronix mx66l51235l on F769-disco, flags
SPI_FLASH_MACRONIX must be enabled.

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