intel/common/smbus: increase spd read performance

This change increases the spd read performance by using smbus word
access.

BUG=b:67021853
TEST=boot to os and find 80~100 ms boot time improvement on one dimm

Change-Id: I98fe67642d8ccd428bccbca7f6390331d6055d14
Signed-off-by: Kane Chen <kane.chen@intel.com>
Reviewed-on: https://review.coreboot.org/22072
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/Kconfig

@@ -1212,6 +1212,9 @@ config DIMM_SPD_SIZE
 	  Total SPD size that will be used for DIMM.
 	  Ex: DDR3 256, DDR4 512.
 
+config SPD_READ_BY_WORD
+	bool
+
 config BOARD_ID_AUTO
 	bool
 	default n