pwm: meson: Remove unneeded platform MODULE_ALIAS

The Amlogic Meson is a DT-only platform, which means the devices are
registered via OF and not using the legacy platform devices support.

So there's no need to have a MODULE_ALIAS("platform:meson-pwm") since
the reported uevent MODALIAS to user-space will always be the OF one.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>