to obtains pv_addr_t structures for the proc0 L1 page table and the proc0
pagetable that maps page tables.
Use the virtual address from the pv_addr_t structure for the proc0 L1 page
table when calling pmap_bootstrap rather than relying on a special mapping
at PAGE_DIRS_BASE.
Modify ofw_construct_proc0_addrspace() for similar changes to
ofw_configmem().
Remove the kernel_pde page table in ofw_construct_proc0_addrspace() as it
it now longer needed.
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