media: vsp1: wpf: Add writeback support

Add support for the writeback feature of the WPF, to enable capturing
frames at the WPF output for display pipelines. To enable writeback the
vsp1_rwpf structure mem field must be set to the address of the
writeback buffer and the writeback field set to true before the WPF
.configure_stream() and .configure_partition() are called. The WPF will
enable writeback in the display list for a single frame, and writeback
will then be automatically disabled.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>