dt-bindings: clock: r8a7779: Add PWM module clock

Add the module clock used by the PWM Timers on the Renesas R-Car H1
(R8A7779) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1397b517fccbe716a71cfae770512ed577730a25.1679329211.git.geert+renesas@glider.be