arch/io.h: Add missing includes

Fixes indirect includes that would break with followup work.

Change-Id: I37ca01b904a0b422a4d09475377e755e167a6ab3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/31697
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/include/cpu/amd/vr.h

@@ -8,6 +8,7 @@
 #define CPU_AMD_VR_H
 
 #include <stdint.h>
+#include <arch/io.h>
 
 #define VRC_INDEX				0xAC1C	// Index register
 #define VRC_DATA				0xAC1E	// Data register