dts: bindings: stm32 qspi description in case of dmamux

Give precision to the description of the dma phandle in the
quadspi node.
When a DMAMUX is present and enabled, the channel is the dma one
(not dmamux channel) and the request is given by the DMAMUX.

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