pwm: mediatek: Add MT7628 support

Add support for MT7628. The SoC is legacy MIPS and hence has no complex
clock tree. This patch add an extra flag to the SoC specific data
indicating, that no clocks are present.

Signed-off-by: John Crispin <john@phrozen.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>