ARM: dts: r8a7791: Add PWM device nodes

Enable support for the 7 PWM channels provided by PWM Timers on R-Car
M2-W, by adding device nodes describing the PWM Timers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200316102540.28887-3-geert+renesas@glider.be