arch/riscv: add missing endian.h header to io.h

Make it uniform as other architectures also include it in io.h

Change-Id: I62c2d909c703f01cdaabdaaba344f82b6746f094
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/28601
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/riscv/include/arch/io.h

@@ -14,6 +14,7 @@
 #ifndef _ASM_IO_H
 #define _ASM_IO_H
 
+#include <endian.h>
 #include <stdint.h>
 #include <compiler.h>