dmaengine: dw-edma: Simplify the DebugFS context CSRs init procedure

DW eDMA v4.70a and older have the read and write channels context CSRs
indirectly accessible. It means the CSRs like Channel Control, Xfer size,
SAR, DAR and LLP address are accessed over at a fixed MMIO address, but
their reference to the corresponding channel is determined by the Viewport
CSR. In order to have a coherent access to these registers the CSR IOs are
supposed to be protected with a spin-lock. DW eDMA v4.80a and newer
normally have unrolled Read/Write channel context registers. That is all
CSRs denoted before are directly mapped in the controller MMIO space.

Since both normal and viewport-based registers are exposed via the DebugFS
nodes, the original code author decided to implement an algorithm based on
the unrolled CSRs mapping with the viewport addresses recalculation if
it's required. The problem is that such implementation turned to be first
unscalable (supports a platform with only single eDMA available since a
base address statically preserved) and second needlessly overcomplicated
(it loops over all Rd/Wr context addresses and re-calculates the viewport
base address on each DebugFS node access). The algorithm can be greatly
simplified just by adding the channel ID and it's direction fields in the
eDMA DebugFS node descriptor. These new parameters can be used to find a
CSR offset within the corresponding channel registers space. The DW eDMA
DebugFS node getter afterwards will also use them in order to activate the
respective context CSRs viewport before reading data from the specified
register. In case of the unrolled version of the CSRs mapping there won't
be any spin-lock taken/released, no viewport activation as before this
modification.

Note this modification fixes the REGISTER() macros using an externally
defined local variable. The same problem with the rest of the macro will
be fixed in the next commit.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>