x86: update iomem_resource end based on CPU physical address capabilities

The iomem_resource map reflects the available physical address space.
We statically initialize the end to -1, i.e., 0xffffffff_ffffffff, but
of course we can only use as much as the CPU can address.

This patch updates the end based on the CPU capabilities, so we don't
mistakenly allocate space that isn't usable, as we're likely to do when
allocating from the top-down.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>