v4l: vsp1: Add support for the BRS entity

The Blend/ROP Sub Unit (BRS) is a stripped-down version of the BRU found
in several VSP2 instances. Compared to a regular BRU, it supports two
inputs only, and thus has no ROP unit.

Add support for the BRS by modelling it as a new entity type, but reuse
the vsp1_bru object underneath. Chaining the BRU and BRS entities seems
to be supported by the hardware but isn't implemented yet as it isn't
the primary use case for the BRS.

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