dts: bindings: introduce a new compatible for stm32 xSPI flash controller

The new bindings for the stm32 xspi is for new stm32 devices with
XSPI peripherals like the stm32h5 serie. This is close to the octo-spi.
Adapt the flash controller constants to the XSPI model especially.
This is done through a new xspi.h definition file.

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