include: drivers: stm32 dma: Provide a GET_INSTANCE macro

To simplify the work on client drivers, provide a STM32_DMA_GET_INSTANCE()
macro which abstracts:
- STM32_DMA_STREAM_OFFSET
- __LL_DMA_GET_STREAM_INSTANCE() vs __LL_DMA_GET_CHANNEL_INSTANCE()

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>