xen-platform: Constify dev_pm_ops

xen-platform: Constify dev_pm_ops

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@gmail.com>
Link: https://lore.kernel.org/r/20200509134755.15038-1-rikard.falkeborn@gmail.com
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>