include: dma: smartbond: Correct include-guard

Both `include/zephyr/drivers/dma/dma_smartbond.h` and
`include/zephyr/dt-bindings/dma/dma_smartbond.h` define `DMA_SMARTBOND_H_`
to prevent duplicate inclusion, so it cannot be properly prevented.

Change to a file path name-based definition.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>