google/gru: Tuning USB 2.0 PHY0 and PHY1 squelch detection threshold

According to USB 2.0 Spec Table 7-7, the High-speed squelch
detection threshold Min 100mV and Max 150mV, and we set USB
2.0 PHY0 and PHY1 squelch detection threshold to 150mV by
default, so if the amplitude of differential voltage envelope
is < 150 mV, the USB 2.0 PHYs envelope detector will indicate
it as squelch.

On Kevin board, if we connect usb device with Samsung U2 cable,
we can see that the impedance of U2 cable is too big according
to the eye-diagram test report, and this cause serious signal
attenuation at the end of receiver, the amplitude of differential
voltage falls below 150mV.

This patch aims to reduce the PHY0 and PHY1 otg-ports squelch
detection threshold to 125mV (host-ports still use 150mV by
default), this is helpful to increase USB 2.0 PHY compatibility.

BRANCH=gru
BUG=chrome-os-partner:62320
TEST=Plug Samsung U2 cable + SEC P3 HDD 500GB/Galaxy S3 into
Type-C port, check if the USB device can be detected.

Change-Id: Ia0a2d354781c2ac757938409490f7c4eecdffe61
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 7d74311c25762668386061234df0562f84b7203e
Original-Change-Id: Ib20772f8fc2484d34c69f5938818aaa81ded7ed8
Original-Signed-off-by: William wu <wulf@rock-chips.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/431015
Original-Commit-Ready: Caesar Wang <wxt@rock-chips.com>
Original-Tested-by: Inno Park <ih.yoo.park@samsung.com>
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/18462
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins)

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