device/dram: Add LPDDR4 utilities

Add lpddr4.c utility file with lpddr4_speed_mhz_to_reported_mts.
Fill in lpddr4_speeds using JDEC 209-4C table 210.
LPDDR4 SPD decoding utilities are not included since there isn't
a present need.

BUG=b:184124605
TEST=Build and run on guybrush

Change-Id: Id8ddfc98fff4255670c50e1ddd4d0a1326265772
Signed-off-by: Rob Barnes <robbarnes@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52745
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/device/dram/lpddr4.h

@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef DEVICE_DRAM_LPDDR4_H
+#define DEVICE_DRAM_LPDDR4_H
+
+/**
+ * @file lpddr4.h
+ *
+ * \brief Utilities for decoding LPDDR4 info
+ */
+
+#include <device/dram/common.h>
+#include <types.h>
+
+/**
+ * Converts LPDDR4 clock speed in MHz to the standard reported speed in MT/s
+ */
+uint16_t lpddr4_speed_mhz_to_reported_mts(uint16_t speed_mhz);
+
+#endif /* DEVICE_DRAM_LPDDR4_H */