dmaengine: pl08x: allocate OF slave channel data at probe time

The current OF translation of channels can never work with
any DMA client using the DMA channels directly: the only way
to get the channels initialized properly is in the
dma_async_device_register() call, where chan->dev etc is
allocated and initialized.

Allocate and initialize all possible DMA channels and
only augment a target channel with the periph_buses at
of_xlate(). Remove some const settings to make things work.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>
Tested-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>