dts: bindings: define the bindings for SPI dma client on stm32 series

This includes the dma cells as client for the spi
An example of the client part is defined in the dma.yaml
Consequently, bindings for the dmas property becomes then 'not required'
Including dma.yaml is not needed as dma properties
are now part of base.yaml

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