of: provide inline helper for of_find_device_by_node

The ipmmu-vmsa driver fails in compile-testing on non-OF platforms:

drivers/iommu/ipmmu-vmsa.o: In function `ipmmu_of_xlate':
ipmmu-vmsa.c:(.text+0x740): undefined reference to `of_find_device_by_node'

It would be reasonable to assume that this interface works but
returns failure on non-OF builds, like it does on machines that
have been booted in another way, so this adds another inline
function helper.

Fixes: 7b2d5961 ("iommu/ipmmu-vmsa: Replace local utlb code with fwspec ids")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <robh@kernel.org>

include/linux/of_platform.h

@@ -57,7 +57,14 @@ extern const struct of_device_id of_default_bus_match_table[];
 extern struct platform_device *of_device_alloc(struct device_node *np,
 					 const char *bus_id,
 					 struct device *parent);
+#ifdef CONFIG_OF
 extern struct platform_device *of_find_device_by_node(struct device_node *np);
+#else
+static inline struct platform_device *of_find_device_by_node(struct device_node *np)
+{
+	return NULL;
+}
+#endif
 
 /* Platform devices and busses creation */
 extern struct platform_device *of_platform_device_create(struct device_node *np,