Some Timer Unit (TMU) instances with 3 channels support a fourth
interrupt: an input capture interrupt for the third channel.

While at it, document the meaning of the four interrupts, and add
"interrupt-names" for clarity.

Update the example to match reality.

Inspired by a patch by Yoshinori Sato for SH.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/8cb38b5236213a467c6c0073f97ccc4bfd5a39ff.1706717378.git.geert+renesas@glider.be