device/dram/ddr2.c: Add methods to compute to identify dram

DDR2 DIMMs are uniquely defined by SPD byte 64 till 72 and 93 till
98. Compute a crc16 over that data to provide a solid way to check
DIMM identify.

Reuse the crc16 function from ddr3.c to do this.

Change-Id: I3c0c42786197f9b4eb3e42261c10ff5e4266120f
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/23345
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/include/device/dram/common.h

@@ -67,4 +67,6 @@ enum spd_status {
 	SPD_STATUS_INVALID_FIELD,
 };
 
+u16 ddr3_crc16(const u8 *ptr, int n_crc);
+
 #endif /* DEVICE_DRAM_COMMON_H */

src/include/device/dram/ddr2.h

@@ -179,5 +179,6 @@ int spd_decode_ddr2(struct dimm_attr_ddr2_st *dimm, u8 spd[SPD_SIZE_MAX_DDR2]);
 void dram_print_spd_ddr2(const struct dimm_attr_ddr2_st *dimm);
 void normalize_tck(u32 *tclk);
 u8 spd_get_msbs(u8 c);
+u16 spd_ddr2_calc_unique_crc(const u8 *spd, int len);
 
 #endif /* DEVICE_DRAM_DDR2L_H */