Run OpenSBI on Mocha

[ziuziakowska]

Tracking issue for running OpenSBI on Mocha.

Currently, I have cloned the CHERI Alliance fork of OpenSBI and have added drivers for the necessary HW blocks (UART, Timer, and PLIC) on my mocha-devel branch: https://github.com/ziuziakowska/opensbi/tree/mocha-devel.

OpenSBI boots and runs the test payload in FW_PAYLOAD mode successfully, but takes about 30 minutes to do so.

Some requirements for integrating into the Mocha repository:

• Figure out how to use cheribuild with CMake to build OpenSBI.
• Build a devicetree blob using dtc for inclusion in ROM/first stage bootloader as that needs to be passed in to describe the hardware for OpenSBI (and all further stages).
• Atomic support is required: Atomics support for AXI #393, currently my fork has patched out any spinlocks and atomics, but this would be infeasible to do for a whole operating system.

[ziuziakowska]

OpenSBI building in: #403, with #392 we can load the built fw_payload.elf/fw_jump.elf into DRAM.