dmaengine: stm32-dma: introduce 3 helpers to address channel flags

Channels 0 to 3 flags are described in DMA_LISR and DMA_LIFCR (L as Low).
Channels 4 to 7 flags are described in DMA_HISR and DMA_HIFCR (H as High).
Macro STM32_DMA_ISR(n) returns the interrupt status register offset for the
channel id (n).
Macro STM32_DMA_IFCR(n) returns the interrupt flag clear register offset
for the channel id (n).

If chan->id % 4 = 2 or 3, then its flags are left-shifted by 16 bits.
If chan->id % 4 = 1 or 3, then its flags are additionally left-shifted by 6
bits.
If chan->id % 4 = 0, then its flags are not shifted.
Macro STM32_DMA_FLAGS_SHIFT(n) returns the required shift to get or set the
channel flags mask.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20220829154646.29867-2-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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