pwm: meson: Add documentation to the driver

Add links to the datasheet and a short summary how the hardware works.
The goal is to make it easier for other developers to understand why the
pwm-meson driver is implemented the way it is.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Co-authored-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>