dmaengine: switchtec-dma: Implement descriptor submission

dmaengine: switchtec-dma: Implement descriptor submission

On prep, a spin lock is taken and the next entry in the circular buffer
is filled. On submit, the spin lock just needs to be released as the
requests are already pending.

When switchtec_dma_issue_pending() is called, the sq_tail register
is written to indicate there are new jobs for the dma engine to start
on.

Pause and resume operations are implemented by writing to a control
register.

Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Co-developed-by: George Ge <george.ge@microchip.com>
Signed-off-by: George Ge <george.ge@microchip.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://patch.msgid.link/20260302210419.3656-4-logang@deltatee.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>