esp: remove pdma_origin from ESPState

Now that all data is transferred via the FIFO (ti_buf) there is no need to track
the source buffer being used for the data transfer. This also eliminates the
need for a separate subsection for PDMA state migration.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-30-mark.cave-ayland@ilande.co.uk>