riscv: Construct an identity map in locore.S

[mhorne]

Summary

This is useful for two reasons. Within this change, it allows the
early DTB mapping to be eliminated, as we can now just dereference the
physical address provided by FW and copy the DTB contents into KVA.

It will also aid in an upcoming change.

[markj]

sys/riscv/riscv/locore.S

124

A more elaborate comment would be helpful:

• When is this map used?
• Is the mapping temporary? If so, when is it removed?
• What does it cover?

sys/riscv/riscv/pmap.c

698

Do you still need this fence? It looks like belongs in pmap_bootstrap_l3().

[markj]

sys/riscv/riscv/locore.S

124

I see that you do this in a follow-up commit.

[mhorne]

sys/riscv/riscv/pmap.c

698

Agreed, it is odd here, but precedes the addition of the code I am removing.

It would be better to check the usage of this fence in the later reworking of pmap_bootstrap(), D45327.

This change is essentially part of that rework, and although I separated it for the sake of review, the split is imperfect.

[markj]

sys/riscv/riscv/locore.S

130

[mhorne]

Add assembly commentary here, not in the later patch.