Add PDMA driver, sf-pdma, to enable DMA engine on HiFive Unleashed
Rev A00 board.

- Implement dmaengine APIs, support MEM_TO_MEM async copy.
- Tested by DMA Test client
- Supports 4 channels DMA, each channel has 1 done and 1 err
interrupt connected to platform-level interrupt controller (PLIC).
- Depends on DMA_ENGINE and DMA_VIRTUAL_CHANNELS

The datasheet is here:

https://static.dev.sifive.com/FU540-C000-v1.0.pdf

Follow the DMAengine controller doc,
"./Documentation/driver-api/dmaengine/provider.rst" to implement DMA
engine. And use the dma test client in doc,
"./Documentation/driver-api/dmaengine/dmatest.rst", to test.

Each DMA channel has separate HW regs and support done and error ISRs.
4 channels share 1 done and 1 err ISRs. There's no expander/arbitrator
in DMA HW.

------ ------
| |--< done 23 >--|ch 0|
| |--< err 24 >--| | (dma0chan0)
| | ------
| | ------
| |--< done 25 >--|ch 1|
| |--< err 26 >--| | (dma0chan1)
|PLIC| ------
| | ------
| |--< done 27 >--|ch 2|
| |--< err 28 >--| | (dma0chan2)
| | ------
| | ------
| |--< done 29 >--|ch 3|
| |--< err 30 >--| | (dma0chan3)
------ ------

Signed-off-by: Green Wan
Link: https://lore.kernel.org/r/20191107084955.7580-4-green.wan@sifive.com
Signed-off-by: Vinod Koul