ARM: dts: r8a7790: Add PWM device nodes

Add support for the 7 PWM channels provided by PWM Timers on R-Car H2,
by adding device nodes describing the PWM Timers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/9755b3af4296060ee31c4652def639574cbbd2fb.1679330878.git.geert+renesas@glider.be