ASoC: sprd: Add reserved DMA memory support

For Spreadtrum audio platform driver, it need allocate a larger DMA buffer
dynamically to copy audio data between userspace and kernel space, but that
will increase the risk of memory allocation failure especially the system
is under heavy load situation.

To make sure the audio can work in this scenario, we usually reserve one
region of memory to be used as a shared pool of DMA buffers for the
platform component. So add of_reserved_mem_device_init_by_idx() function
to initialize the shared pool of DMA buffers to be used by the platform
component.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>