dma_dw: separate intel specific registers from synopsys ones

CFG register uses fields that are not defined in Synopsys databook of
Designware AHB DMA Controller.

Since current Zephyr code uses this driver only for the
intel_adsp_gpdma driver I assume that those fields are specific to
this DMA which is not the standard Designware one.

This patch allows to use either the standard Designware register or
the Intel one.

Signed-off-by: Sylvain Chouleur <schouleur@graimatterlabs.ai>