dmaengine: dw: Remove misleading is_private property

The commit a9ddb575

   ("dmaengine: dw_dmac: Enhance device tree support")

introduces is_private property in uncertain understanding what does it mean.

First of all, documentation defines DMA_PRIVATE capability as

Documentation/crypto/async-tx-api.txt:
  The DMA_PRIVATE capability flag is used to tag dma devices that should not be
  used by the general-purpose allocator. It can be set at initialization time
  if it is known that a channel will always be private. Alternatively,
  it is set when dma_request_channel() finds an unused "public" channel.

  A couple caveats to note when implementing a driver and consumer:
  1/ Once a channel has been privately allocated it will no longer be
     considered by the general-purpose allocator even after a call to
     dma_release_channel().
  2/ Since capabilities are specified at the device level a dma_device with
     multiple channels will either have all channels public, or all channels
     private.

Documentation/driver-api/dmaengine/provider.rst:
  - DMA_PRIVATE
    The devices only supports slave transfers, and as such isn't available
    for async transfers.

The capability had been introduced by the commit 59b5ec21

  ("dmaengine: introduce dma_request_channel and private channels")

and some code didn't changed from that times ever.

Taking into consideration above and the fact that on all known platforms
Synopsys DesignWare DMA engine is attached to serve slave transfers,
the DMA_PRIVATE capability must be enabled for this device unconditionally.
Otherwise, as rightfully noticed in drivers/dma/at_xdmac.c:
  /*
   * Without DMA_PRIVATE the driver is not able to allocate more than
   * one channel, second allocation fails in private_candidate.
   */
because of of a caveats mentioned in above documentation excerpts.

So, remove conditional around DMA_PRIVATE followed by removal leftovers.

If someone wonders, DMA_PRIVATE can be not used if and only if the all channels
of the DMA controller are supposed to serve memory-to-memory like operations.
For example, EP93xx has two controllers, one of which can only perform
memory-to-memory transfers

Note, this change doesn't affect dmatest to be able to test such controllers.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> (maintainer:SERIAL DRIVERS)
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>