dt-bindings: clock: renesas,cpg-mssr: Document r8a779f0

Add binding documentation for the R-Car S4-8 (R8A779F0) Clock Pulse
Generator.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20211116074130.107554-7-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>