ARM: dts: r8a7791: Add TPU device node

Enable support for the 4 PWM channels provided by the 16-bit Timer Pulse
Unit on R-Car M2-W, by adding a device node describing the TPU.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200316102540.28887-2-geert+renesas@glider.be