rtc: Add an RTC API, and implement it for x86.

This CL adds an API for RTC drivers, and implements its two functions,
rtc_get and rtc_set, for x86's RTC. The function which resets the clock when the
CMOS as lost state now uses the RTC driver instead of accessing the those
registers directly.

BUG=None
TEST=Built and booted on Link with the event log code modified to use
the RTC interface. Verified that the event times were accurate.
BRANCH=nyan

Original-Change-Id: Ifa807898e583254e57167fd44932ea86627a02ee
Original-Signed-off-by: Gabe Black <gabeblack@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/197795
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
Original-Tested-by: Gabe Black <gabeblack@chromium.org>
Original-Commit-Queue: Gabe Black <gabeblack@chromium.org>

This is the first half of the patch.
(cherry picked from commit 9e0fd75142d29afe34f6c6b9ce0099f478ca5a93)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: I159f9b4872a0bb932961b4168b180c087dfb1883
Reviewed-on: http://review.coreboot.org/7889
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/include/bcd.h

@@ -0,0 +1,35 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright 2014 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.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA, 02110-1301 USA
+ */
+
+#ifndef _BCD_H_
+#define _BCD_H_
+
+#include <stdint.h>
+
+static inline uint8_t bcd2bin(uint8_t val)
+{
+	return ((val >> 4) & 0xf) * 10 + (val & 0xf);
+}
+
+static inline uint8_t bin2bcd(uint8_t val)
+{
+	return ((val / 10) << 4) | (val % 10);
+}
+
+#endif /* _BCD_H_ */