rockchip: rk3399: add simplest sdram to fix compiling error

This patch is only to make building happy, the real sdram driver
comes later.

BRANCH=none
BUG=none
TEST=emerge-kevin coreboot

Change-Id: I4123c3a6627d7264c615fefbb89e16c4dfb9a423
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 5b992a7895a72c83f57228d3abd1ae37d55e7e7b
Original-Change-Id: Ie340877e828ae760169ccfa9a7099e7472d2fc26
Original-Signed-off-by: Shunqian Zheng <zhengsq@rock-chips.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/338944
Original-Commit-Ready: Vadim Bendebury <vbendeb@chromium.org>
Original-Tested-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://review.coreboot.org/14703
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/soc/rockchip/rk3399/sdram.c

@@ -0,0 +1,21 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright 2016 Rockchip 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.
+ */
+
+#include <soc/sdram.h>
+
+size_t sdram_size_mb(void)
+{
+	return CONFIG_DRAM_SIZE_MB;
+}