dt-bindings: pwm: renesas: tpu: Document R8A779{7|8}0 bindings

Document the R-Car V3{M|H} (R8A779{7|8}0) SoC in the Renesas TPU
bindings; the TPU hardware in those is the Renesas standard 4-channel
timer pulse unit.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>