imxrt_flexpwm independent output B support added

[jturnsek]

Summary

i.MXRT FlexPWM has two outputs per PWM submodule, called A and B, respectively. Output B can be set to be complementary output for Output A or independent. Later is increasing the number of channels per FlexPWM module from 4 to 8 if CONFIG_PWM_MULTICHAN is used. This PR is supporting this independent mode. If complementary mode is used, channel number of Output A must be used.

Impact

CONFIG_PWM_NCHANNELS must be set to 8 (maximum) and numbering of channels must be according, i.e. 1 to 8. Even if submodules are excluded with configuration. THis means if only MOD3 is used in FlexPWM1, usable channels are 5 and 6.

Testing

Tested with rgbled example on EdgePro1, where FlexPWM2 MOD3 Out B is used for Red and FlexPWM1 MOD4 Out A and Out B is used for Green and Blue.

[acassis]

@jturnsek does that complementary PWM has dead time support to H-Bridge usage like Michal submitted to SAM MCU?
More info: https://community.element14.com/technologies/fpga-group/b/blog/posts/vhdl-pwm-generator-with-dead-time-the-design

[jturnsek]

does that complementary PWM has dead time support to H-Bridge usage like Michal submitted to SAM MCU? More info: https://community.element14.com/technologies/fpga-group/b/blog/posts/vhdl-pwm-generator-with-dead-time-the-design

FlexPWM actually has a dead time support, but I think it is not used here. That is something to be considered in future improvements.

[michallenc]

@acassis I think this can be merged if you agree.