ofw_read_mem_regions() can skip calculation of physmem. pmap.c

ofw_read_mem_regions() can skip calculation of physmem.  pmap.c
already calculates _usable_ memory and updates physmem (if it is 0),
whereas ofw_read_mem_regions() was counting usable+unuseable memory.
ie. 4G or more on some machines. powerpc's 32-bit pagetable cannot use memory
beyond 4G phys addr.
(On a 4G machine, physmem64 was calculated as 0, which caused the installer's
auto-diskabel code to place /tmp on the b partition).
ok gkoehler, works for kurt also