xtensa: Fix build error due to missing include file

Commit 69d99e6c ("xtensa: Remove unnecessary of_platform_populate
with default match table") dropped various include files from
arch/xtensa/kernel/setup.c. This results in the following build error.

arch/xtensa/kernel/setup.c: In function ‘xtensa_dt_io_area’:
arch/xtensa/kernel/setup.c:213:2: error:
	implicit declaration of function ‘of_read_ulong’

Fixes: 69d99e6c ("xtensa: Remove unnecessary of_platform_populate with default match table")
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

arch/xtensa/kernel/setup.c

@@ -23,6 +23,7 @@
 #include <linux/kernel.h>
 #include <linux/percpu.h>
 #include <linux/cpu.h>
+#include <linux/of.h>
 #include <linux/of_fdt.h>
 
 #if defined(CONFIG_VGA_CONSOLE) || defined(CONFIG_DUMMY_CONSOLE)