samples: drivers: stm32u5x octospi enables DMA

Adds the dma transfer for the octoSPI on NOR octo Flash
of the b_u585i_iot02a disco kit.
The channel for the GPDMA is 0-15 (0-7 for DMA1, 8-15 for DMA2)
The channel 12 to 15 are used for GPDMA transfers
to/from external memories.
The request is 41 for the OCTOSPI2.

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