ARM: dts: vcu: define indexes for output clocks

The VCU System-Level Control has 4 output clocks. Define indexes for
these clocks to allow to reference them in the device tree.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20210121071659.1226489-2-m.tretter@pengutronix.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>