mb/siemens/mc_ehl1: Disable L1 prefetcher

The highly real time driven application executed on mc_ehl1 has shown
that the L1 prefetcher on Elkhart Lake is too aggressive which in the
end leads to an increased number of cache misses. Disabling the L1
prefetcher boosts up the performance (in some cases by more than 10 %)
in this specific use case.

Change-Id: Id09a7f8f812707cd05bd5e3b530e5c3ad8067b16
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68668
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>

src/mainboard/siemens/mc_ehl/variants/mc_ehl1/devicetree.cb

@@ -132,6 +132,9 @@ chip soc/intel/elkhartlake
 		.vcc_low_high_us = 50,
 	}"
 
+	# Disable L1 prefetcher
+	register "L1_prefetcher_disable" = "true"
+
 	device domain 0 on
 		device pci 00.0 on	end # Host Bridge
 		device pci 02.0 on	end # Integrated Graphics Device