dartmonkey: Note for unused SLP_S4_L and SLP_SUS_L
dartmonkey: Note for unused SLP_S4_L and SLP_SUS_L

These sleep lines were never used in code and have since been removed
from the reference schematic, but they are driven on nocturne.
This means that it would be bad if thee lines were ever accidentally
repurposed in the future.
Confirmed on A70 PVT/MP schematic.

TEST=make BOARD=dartmonkey
BUG=b:175115925

Change-Id: I3d1e3bbee5d242026a37464f6f0154243e262972
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2579463
Tested-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Auto-Submit: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Commit-Queue: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec