[media] v4l: vsp1: Make number of BRU inputs configurable

The R-Car Gen3 family has 5-inputs BRUs, support them by making the
number of BRU inputs configurable.

As the driver assumes that the number of BRU inputs is equal to the
number of RPFs, replace the BRU_MAX_INPUTS macro with VSP1_MAX_RPF to
make the assumption apparent.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>