intel/skylake: Add devicetree setting for DDR frequency limit UPD

intel/skylake: Add devicetree setting for DDR frequency limit UPD

There is a UPD setting exposed by FSP that allows the DDR
frequency to be limited.  Expose this for devicetree.

BUG=chrome-os-partner:47346
BRANCH=none
TEST=tested by limiting DDR frequency to 1600 on chell EVT

Change-Id: I1f17b221d9fa4c2dd1e8c5f403deb0f2bc0493a7
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 91f760ad19823225f7e5bd2dc690164ed253e220
Original-Change-Id: Ibcd4a65a9cfd7d32fbf2ba8843ab25da8e9cf28a
Original-Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/317243
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/12981
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>