configs: stm32f469-disco: Enable QSPI relative flags

Enable CMD_SF, MTD, DM_SPI_FLASH, SPI_FLASH, SPI, DM_SPI and
STM32_QSPI flags to be able to use the embedded n25q128a QSPI flash
on stm32f469-disco board.

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