Allow cacheable mmaps of legacy_mem if WB access is supported for the region.
The "legacy_mem" file often contains a shadow option ROM, and some versions of
X depend on this.

Tim Yamin <plasm@roo.me.uk> reported that this change fixes X on a Dell
PowerEdge 3250.
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>