RISC-V SBI Specification, release candidate for v0.3

This tag contains the first release candidate of version 0.3.0 of the RISC-V SBI specification. It includes few document styling improvements and following new extensions.

• System reset
• HART suspend
• Performance Monitoring Unit (PMU)

We have tagged this as a release candidate for a wider review. We don't expect any more functional changes in the v0.3.
However, cosmetic and/or semantic changes still can be included if found. We are planning to release the v0.3 in a month time if everything goes well.