nds32: Add a basic errno.h

This is available on other architectures, and nds32 will start to break
without it as code starts to use error numbers more.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/nds32/include/asm/errno.h

@@ -0,0 +1 @@
+#include <asm-generic/errno.h>