Although not strictly required to boot a 64MB board, bump
vm_max_virtual_address to be KERNVIRTADDR + 256MB. This allows some
future shock protection since the KVA requirements have gone up since
the unmapped changes have gone in, as well as preventing us from
overlapping with the hardware devices, which we map at 0xd0000000,
which we'd hit with anything more than 85MB...

MFC after:	3 days