riscv: Add a stub pmap_change_attr implementation

pmap_change_attr is required by drm-kmod so we need the function to
exist. Since the Svpbmt extension is on the horizon we will likely end
up with a real implementation of it, so this stub implementation does
all the necessary page table walking to validate the input, ensuring
that no new errors are returned once it's implemented fully (other than
due to out of memory conditions when demoting L2 entries) and providing
a skeleton for that future implementation.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31996