dma4channel: Support extended DMA engines on Raspberry Pi 5

* 6 channels available (6-11)
* Only memory copy and interrupt has been tested so far
* Normal and lite DMA engines are not supported yet
* machineinfo: AllocateDMAChannel() always returns extended channel
* machineinfo: DMA channel map should be requested from DTB
* sysconfig: Re-enable SCREEN_DMA_BURST_LENGTH for Raspberry Pi 5