mb/google/auron: Factor out SPD indexing

The code to read the SPD file and index it is not variant-specific.

Change-Id: Iaee0a77934a45c65bf32dd0dba23cec654abc0b9
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49773
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/mainboard/google/auron/variant.h

@@ -13,7 +13,7 @@ void variant_romstage_entry(struct romstage_params *rp);
 void lan_init(void);
 
 void mainboard_fill_spd_data(struct pei_data *pei_data);
-void mainboard_print_spd_info(uint8_t spd[]);
+void fill_spd_for_index(uint8_t spd[], unsigned int index);
 
 #define SPD_LEN			256