Implement support for getting hardware randomness on RISC-V

RISC-V random number generation is not currently supported. Here are some notes from @andrey.turkin regarding random number generation instructions in RISC-V: None exist in CPU AFAIK. However non-cpu sources might also exist, e.g. there is a standard /chosen/rng-seed node in DT which can be populated by a platform-specific firmware (https://github.com/devicetree-org/dt-schema/blob/19d754e9e2b6b319bbfd98efa26c1578e61a088a/schemas/chosen.yaml#L55), UEFI has random number generator protocol so bootloader can tap into that and pass it along in env etc