sun4v: add cbus driver - from OpenBSD

On SUN4V make the mdesc data available early in pmap_bootstrap, so we
can set up the uvm page colors correctly.
Based on code from palle@.

sun4v: added new function mdesc_name_by_idx() for retrieving the name of a node

sun4v: Add two new functions to the mdesc api: mdesc_find_node_by_idx() and mdesc_next_node()

sun4v: add hypervisor machine description (mdesc) functions - from OpenBSD