PCI: Add arch_can_pci_mmap_wc() macro

Most of the almost-identical versions of pci_mmap_page_range() silently
ignore the 'write_combine' argument and give uncached mappings.

Yet we allow the PCIIOC_WRITE_COMBINE ioctl in /proc/bus/pci, expose the
'resourceX_wc' file in sysfs, and allow an attempted mapping to apparently
succeed.

To fix this, introduce a macro arch_can_pci_mmap_wc() which indicates
whether the platform can do a write-combining mapping.  On x86 this ends up
being pat_enabled(), while the few other platforms that support it can just
set it to a literal '1'.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

include/linux/pci.h

@@ -1626,6 +1626,10 @@ static inline int pci_get_new_domain_nr(void) { return -ENOSYS; }
 
 #include <asm/pci.h>
 
+#ifndef arch_can_pci_mmap_wc
+#define arch_can_pci_mmap_wc()		0
+#endif
+
 #ifndef pci_root_bus_fwnode
 #define pci_root_bus_fwnode(bus)	NULL
 #endif