dt-bindings: clock: renesas,cpg-mssr: Document r8a779a0

Add binding documentation for the R-Car V3U (R8A779A0) Clock Pulse
Generator.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1599470390-29719-7-git-send-email-yoshihiro.shimoda.uh@renesas.com


Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>