Deduplicate and split platforms

[JohnAZoidberg]

Currently we are working on the following platforms:

• Sifive Hifive Unleashed
• QEMU virt platform
• BeagleV

Currently only the first one is on the main branch and upstreamed. The other two have WIP branches where they don't have their own platform but only the Hifive Unleashed code is modified.

Progress

On branch riscv-virt-gh-actions

• Move generic OpenSbiPlatformLib to Platforms/RISCV/PlatformPkg
• Have RiscvVirt use generic OpenSbiPlatformLib
• Have U540 and U500 use generic OpenSbiPlatformLib
• Deduplicate PlatformPei

- [ ] Follow MinPlatform See #24

[JohnAZoidberg]

OpenSBI platform files are generic now. They're only configured by the device tree.
So we can move the OpenSbiPlatformLib to RISC-V/PlatformPkg.

[JohnAZoidberg]

The DSC and FDF files could be deduplicated by implementing MinPlatform.

[JohnAZoidberg]

E.g. see how it's done for Cometlake: https://github.com/riscv/riscv-edk2-platforms/blob/master/Platform/Intel/CometlakeOpenBoardPkg/CometlakeURvp/OpenBoardPkg.dsc

[JohnAZoidberg]

What's remaining is https://github.com/riscv/riscv-edk2-platforms/blob/master/Platform/SiFive/U5SeriesPkg/FreedomU540HiFiveUnleashedBoard/Universal/Pei/PlatformPei/
These are 300 lines of code that could be shared and be configured using a couple of PCDs on each platform.

[JohnAZoidberg]

Done with 36bcc51 and 1442441