Merge https://source.denx.de/u-boot/custodians/u-boot-riscv

* Add RISC-V falcon mode documentation
* Add Clang build support
* Add cmd to detect Debug Trigger Extension support

* Add PWM setting for Unmatched board
* Add Milk-V Duo board support
* Add new device node and enable new config option for VisionFive2 board
* Add second virtio device for RISC-V QEMU