x86: allocate space within a region top-down

Request that allocate_resource() use available space from high addresses
first, rather than the default of using low addresses first.

The most common place this makes a difference is when we move or assign
new PCI device resources.  Low addresses are generally scarce, so it's
better to use high addresses when possible.  This follows Windows practice
for PCI allocation.

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=16228#c42


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