riscv: Port improvements from arm64/amd64 pmaps, part 1

[markj]

Summary

• When promoting, do not require that all PTEs all have PTE_A set. Instead, record whether they did and store this information in the PTP's valid bits.
• Synchronize some comments in pmap_promote_l2().
• Make pmap_promote_l2() scan starting from the end of the 2MB range instead of the beginning. See the commit log for 9d1b7fa31f510 for justification of this, which I believe applies here as well.