[media] v4l: vsp1: Add support for RT clock

The VSPR and VSPS instances use two clocks, the VSP1 system clock and
the VSP1 realtime clock. Both of them need to be enabled to access the
VSP1 registers.
Add support for an optional RT clock and enable/disable it along with
the system clock.

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