riscv: Construct an identity map in locore.S

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: the larger reworking of page
table bootstrapping on this platform.

Reviewed by: markj, jhb
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45324