dev_pm_ops is never modified, so mark it const to allow the compiler to
put it in read-only memory.
Before:
text data bss dec hex filename
2457 1668 256 4381 111d drivers/xen/platform-pci.o
After:
text data bss dec hex filename
2681 1444 256 4381 111d drivers/xen/platform-pci.o
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@xxxxxxxxx>