pwm: mediatek: Remove redundant MODULE_ALIAS entries

MODULE_ALIAS exports information to allow the module to be auto-loaded at
boot for the drivers registered using legacy platform registration.

However, currently the driver is always used by DT-only platform,
MODULE_ALIAS is redundant and should be removed properly.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>