samples: drivers: spi_flash includes the dma for stm32 xspi node

The stm32 xspi node is another possible multi-spi for connecting
with gpDMA channels for Rx and Tx, same request.
Channels 4 and 5 are preferred for AHB periph to/from ext. Mem.

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