mb/starlabs/starbook/mtl: Correct DIMM Speed Size

The DDR5 modules have a speed size of 1024 bytes, not 512. Update
Kconfig to reflect this.

Change-Id: Ic7b691104ff8b0061a485f01709a2f53046cc94a
Signed-off-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86306
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

src/mainboard/starlabs/starbook/Kconfig

@@ -139,6 +139,7 @@ config DEVICETREE
 	default "variants/\$(CONFIG_VARIANT_DIR)/devicetree.cb"
 
 config DIMM_SPD_SIZE
+	default 1024 if BOARD_STARLABS_STARBOOK_MTL
 	default 512
 
 config DRIVER_TPM_SPI_CHIP