RP.PWM: add function to get compare register address

[Fabien-Chouteau]

This is useful for DMA transfers.

[Fabien-Chouteau]

Hi @JeremyGrosser ,

I changed this to return the base address of the 32bit CC register. It's now up to the user to:

• Use 32bit DMA to send values to both channels
• Use 16bit DMA and have the value duplicated on both channels