[build] Centralise dummy architecture-specific headers

Simplify the process of adding a new CPU architecture by providing
common implementations of typically empty architecture-specific header
files.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/bits/pci_io.h

@@ -0,0 +1,15 @@
+#ifndef _BITS_PCI_IO_H
+#define _BITS_PCI_IO_H
+
+/** @file
+ *
+ * Dummy architecture-specific PCI I/O API implementations
+ *
+ * This file is included only if the architecture does not provide its
+ * own version of this file.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#endif /* _BITS_PCI_IO_H */