media: v4l: vsp1: Rename BRU to BRx

Some VSP instances have two blending units named BRU (Blend/ROP Unit)
and BRS (Blend/ROP Sub unit). The BRS is a smaller version of the BRU
with only two inputs, but otherwise offers similar features and offers
the same register interface. The BRU and BRS can be used exchangeably in
VSP pipelines (provided no more than two inputs are needed).

Due to historical reasons, the VSP1 driver implements support for both
the BRU and BRS through objects named vsp1_bru. The code uses the name
BRU to refer to either the BRU or the BRS, except in a few places where
noted explicitly. This creates confusion.

In an effort to avoid confusion, rename the vsp1_bru object and the
corresponding API to vsp1_brx, and use BRx to refer to blend unit
instances regardless of their type. The names BRU and BRS are retained
where reference to a particular blend unit type is needed, as well as in
hardware registers to stay close to the datasheet.

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