boards/raspberrypi-4b: PWM audio support

[linguini1]

Summary

This PR introduces support for PWM-based audio on the Raspberry Pi 4B:

• PWM driver support for both channels of PWM0 and PWM1 (audio jack PWM)
• Oneshot timer support for the (4) BCM2711 system timers
• Device driver registration for PWM and audio tone drivers
• A configuration with the PWM example to test audio, which is documented

The PWM audio isn't the best sounding because it is PWM. There have been suggestions on how to improve NuttX's audio support to allow translating PCM audio to PWM with DMA, but for now that is beyond the scope of these improvements.

Impact

Closes #16950.
Closes #19374.

Part of my GSoC milestones, completing the audio jack support milestone.

Really hurts your ears if you wear earbuds to test; I recommend a device with its own analog volume control knob, and cautiously testing the volume of the sound before committing to wearing earbuds/headphones. I put this warning in the docs, but this is a warning to reviewers too!

Testing

Built the PWM audio configuration that is included in this PR and verified on the device that audio comes through the headphone jack. To my (not perfectly trained ears), the audio frequencies I set for a few notes (i.e. middle C) have sound which matches what my desktop computer played for a recording of middle C I found online.

I also registered the one-shot driver as a character driver and performed the one-shot test, which passed:

nsh> oneshot                                                                                      
oneshot_ioctl: cmd=5664 arg=004de0a0                                                              
oneshot_ioctl: cmd=5665 arg=004de0b0                                                              
bcm2711_tim_irq_en: Interrupt for timer 0 enabled.                                                
Opening /dev/oneshot0                                                                             
Maximum delay is 18446744073709551615                                                             
Starting oneshot timer with delay 2000000 microseconds                                            
Waiting...                                                                                        
bcm2711_tim_irq_en: Interrupt for timer 0 disabled.                                               
bcm2711_tim_handler: Callback...                                                                  
bcm2711_tim_handler: Callback called for oneshot 0.                                               
Finished                                                    

And finally, I checked the PWM0 interface output using my logic analyzer and both the frequency and duty cycle settings seem to work perfectly fine:

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb