Fix offset for pwms with different periods.
The previous version was using an offset scheme based on pwm counter_0.
By using a separate offset counter the user will not be constrained by
pwm_0 period in regards with the offset of other pulses. In this version
offset 0 is used to delay pwm 0 in regards to the offset counter.
The offset counter will start after the load_config signal is asserted
and all active pwm counters finish the previous cycle or by a software
reset.
The software reset should also be used when using external_sync.
81320b6469