dm: x86: Add a gpio header for coreboot

This code doesn't follow the normal approach of having its arch-specific
definitions in an arch-specific directory. Add a new arch-specific file
and make use of it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/include/asm/gpio.h b/arch/x86/include/asm/gpio.h
index fe09f31..8bda414 100644
--- a/arch/x86/include/asm/gpio.h
+++ b/arch/x86/include/asm/gpio.h
@@ -6,6 +6,7 @@
 #ifndef _X86_GPIO_H_
 #define _X86_GPIO_H_
 
+#include <asm/arch/gpio.h>
 #include <asm-generic/gpio.h>
 
 #endif /* _X86_GPIO_H_ */