of: only include prom.h on sparc

The dependency on prom.h by the core DT code is now removed and only
sparc needs to include prom.h for the core code.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>

include/linux/of.h

@@ -136,7 +136,9 @@ static inline unsigned long of_read_ulong(const __be32 *cell, int size)
 	return of_read_number(cell, size);
 }
 
+#if defined(CONFIG_SPARC)
 #include <asm/prom.h>
+#endif
 
 /* Default #address and #size cells.  Allow arch asm/prom.h to override */
 #if !defined(OF_ROOT_NODE_ADDR_CELLS_DEFAULT)