Convert the Spreadtrum SC9860 DMA bindings to DT schema.

Changes during conversion:
  - rename file to match compatible
  - make interrupts optional, the AGCP DMA controller doesn't need it
  - describe the optional ashb_eb clock for the AGCP DMA controller
Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/Zob1+kGW1xeBKehA@standask-GA-A55M-S2HPSigned-off-by: Vinod Koul <vkoul@kernel.org>