dt-bindings: clock: r8a7796: Remove CSIREF clock

The R-Car Gen3 HardWare Manual Errata for Rev. 0.52 (Nov 30, 2016)
removed the CSI reference clock on R-Car M3-W.

As this definition was never used, it can just be removed.
The freed slot in the DT bindings header must not be reused, though.

Fixes: 972610fb ("clk: renesas: Add r8a7796 CPG Core Clock Definitions")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@kernel.org>