dmatest no longer works on ARC SDP with DW DMAC

[Eugeniy Paltsev]

dmatest on ARC SDP with DW DMAC became broken after df5c7386
("dmaengine: dw: some Intel devices has no memcpy support") and
30cb2639 ("dmaengine: dw: don't override platform data with autocfg")
commits.
* After df5c7386 commit "DMA_MEMCPY" capability option doesn't
get set correctly in platform driver version.
* After 30cb2639 commit
"data_width" and "nollp" parameters don't get set correctly in platform
driver version.

This happens because in old driver version there are three sources 
of parameters: pdata, device tree and autoconfig hardware registers. 
Some parameters were read from pdata and others from autoconfig
hardware registers. If pdata was absent some pdata structure 
fields were filled with parameters from device tree. But 30cb2639
commit disabled overriding pdata with autocfg, so if we use platform
driver version without pdata some parameters will not be set.

I'm wondering what would be the best way to fix this situation?
Should we strictly read parameters from only one source (pdata/device
tree/autoconfig) or we may mix some of them (for example getting
missing data from autoconf regs)?