bindings: qspi: stm32 qspi supporting Dual Flash Mode

Configure the quad-spi in DualFlash Mode
This property of the stm32 boards will access simultaneously
two identical quad-flash external memories connected to
2 quad-spi banks (pins).
Dual Flash Mode is possible on stm32 series with QUADSPI_CR_DFM

Signed-off-by: Francois Ramu <francois.ramu@st.com>