timing control not exposed

It is impossible to control relative phase between channels as LEDn_ON and LEDn_OFF registers are not exposed.

Please expose LEDn_ON and LEDn_OFF registers and let user customize their own timing instead of accepting a simple duty cycle value.

Start by locating the PCA9685 driver entry point that currently accepts a duty-cycle value and the handling for LEDn_ON and LEDn_OFF registers. Read how channel timing is represented and check any existing tests or usage examples. Done means users can set both register values and control relative phase between channels.

Written by the indexing model from the issue text.