soc/intel/elkhartlake: Provide a way to enable real-time tuning

Intel provides a Real-Time Tuning Guide for Elkhart Lake to improve
real-time behaviour of the SoC (see Intel doc #640979). It describes,
amongst knobs for the OS, a couple of firmware settings that need to be
set properly to reduce latencies in all the subsystems. Things like
clock and power gating as well as low power states for peripherals and
buses are disabled in this scenario.

This patch takes the mentioned UEFI parameters from the guide and
translates them to FSP-M and FSP-S parameters. In addition, a chip
config switch guards this tuning which can be selected on mainboard
level if needed.

When this real-time tuning is enabled, the overall system performance
in a real-time environment can be increased by 2-3%.

Change-Id: Ib524ddd675fb3ea270bacf8cd06cb628e895b4b6
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71233
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

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