of: add include asm/setup.h in drivers/of/fdt.c

In the file drivers/of/fdt.c, it uses the COMMAND_LINE_SIZE which is stated
in asm/setup.h, so asm/setup.h should be included in drivers/of/fdt.c.
Signed-off-by: Yu Yue <yuyue@mprc.pku.edu.cn>
Signed-off-by: Guan Xuetao <guanxuetao@mprc.pku.edu.cn>

Cc: Grant Likerly <grant.likely@secretlab.ca>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Arnd Bergmann <arnd@arndb.de>

drivers/of/fdt.c

@@ -23,6 +23,7 @@
 #include <asm/machdep.h>
 #endif /* CONFIG_PPC */
 
+#include <asm/setup.h>
 #include <asm/page.h>
 
 char *of_fdt_get_string(struct boot_param_header *blob, u32 offset)