device/dram/ddr3: Move CRC calculation in a separate function

Calculating the CRC of a SPD may be useful by itself, so split that
part of the code in a separate function.

Change-Id: I6c20d3db380551865126fd890e89de6b06359207
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4537
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/device/dram/ddr3.h

@@ -170,6 +170,7 @@ enum spd_status {
 
 typedef u8 spd_raw_data[256];
 
+u16 spd_ddr3_calc_crc(u8 *spd, int len);
 int spd_decode_ddr3(dimm_attr * dimm, spd_raw_data spd_data);
 int dimm_is_registered(enum spd_dimm_type type);
 void dram_print_spd_ddr3(const dimm_attr * dimm);