efi/reboot: Add generic wrapper around EfiResetSystem()

Implement efi_reboot(), which is really just a wrapper around the
EfiResetSystem() EFI runtime service, but it does at least allow us to
funnel all callers through a single location.

It also simplifies the callsites since users no longer need to check to
see whether EFI_RUNTIME_SERVICES are enabled.

Cc: Tony Luck <tony.luck@intel.com>
Tested-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/ia64/kernel/process.c

@@ -662,7 +662,7 @@ void
 machine_restart (char *restart_cmd)
 {
 	(void) notify_die(DIE_MACHINE_RESTART, restart_cmd, NULL, 0, 0, 0);
-	(*efi.reset_system)(EFI_RESET_WARM, 0, 0, NULL);
+	efi_reboot(REBOOT_WARM, NULL);
 }
 
 void