Define a mainboard driver

A mainboard driver will do init functions as needed, as well as
supporting read and write. The ClockNode might be useful.

Signed-off-by: Ronald G Minnich <rminnich@gmail.com>

src/mainboard/amd/romecrb/src/mainboard.rs

@@ -0,0 +1,46 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright (C) 2020 Google Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+pub struct MainBoard {
+}
+
+impl MainBoard {
+    pub fn new(base: usize, baudrate: u32) -> MainBoard {
+        MainBoard { }
+    }
+}
+
+impl Driver for MainBoard {
+    fn init(&mut self) -> Result<()> {
+        Ok(())
+    }
+
+    fn pread(&self, data: &mut [u8], _offset: usize) -> Result<usize> {
+            return Ok(0);
+    }
+
+    fn pwrite(&mut self, data: &[u8], _offset: usize) -> Result<usize> {
+        Ok(data.len())
+    }
+
+    fn shutdown(&mut self) {}
+}
+
+impl ClockNode for MainBoard {
+    // This uses hfclk as the input rate.
+    fn set_clock_rate(&mut self, rate: u32) {
+    }
+}
+