dt-bindings: spi: add stm32 qspi controller

This patch adds the documentation of device tree bindings
for the STM32 QSPI controller. It is a specialized communication
interface targeting single, dual or quad SPI Flash memories (NOR/NAND).

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>