riscv: Add a stub pmap_change_attr implementation

[jrtc27]

Summary

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.

MFC after: 1 week

[jrtc27]

Handle L1 pages as those are uesd for the direct map

QEMU's devices are seemingly all too simple to ever hit the direct map case, but
on real hardware if_cgem quickly trips up on this and panics during attach in
pmap_page_set_memattr.