ARM: keystone: fix missing keystone.h in pm_domain.c

The declaration of keystone_pm_runtime_init() is not included
from keystone.h in pm_domain.c. Including the file fixes the
following sparse warning:

arch/arm/mach-keystone/pm_domain.c:37:12: warning: symbol 'keystone_pm_runtime_init' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>

arch/arm/mach-keystone/pm_domain.c

@@ -18,6 +18,8 @@
 #include <linux/platform_device.h>
 #include <linux/of.h>
 
+#include "keystone.h"
+
 static struct dev_pm_domain keystone_pm_domain = {
 	.ops = {
 		USE_PM_CLK_RUNTIME_OPS