ARM: dts: r8a7790: Add TPU device node

Add support for the 4 PWM channels provided by the 16-bit Timer Pulse
Unit on R-Car H2, by adding a device node describing the TPU.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/75da1e63135a3fc8a3aaafbff7139bd5d7509be3.1679330727.git.geert+renesas@glider.be