dmaengine: axi-dmac: move active_descs list init after device-tree init

dmaengine: axi-dmac: move active_descs list init after device-tree init

We want to enable the clock right after it is obtained. Then later we'll
want to read the core version via register-access (which requires the clock
to be enabled).

The initialization of the active_descs list can be postponed after reading
from registers (or reading the device-tree).

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200825151950.57605-3-alexandru.ardelean@analog.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>