Use pmap_mapdev()/unmapdev() to temporarily map on-chip sram while copying
the startup trampoline code. The old code allocated a kva page, mapped it
using using pmap_kenter_nocache(), then freed the kva without destroying
the mapping. This is the only use of pmap_kenter_nocache() in the system,
so redoing this one use of allows it to be garbage collected in the
near future.