dt-bindings: mtd: Document the STM32 QSPI bindings

This patch adds documentation of device tree bindings for the STM32
QSPI controller.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>