arm: mvebu: a38x: serdes: improve USB3 electrical configuration

arm: mvebu: a38x: serdes: improve USB3 electrical configuration

This is a backport from Marvell U-Boot:
https://github.com/MarvellEmbeddedProcessors/u-boot-marvell
commit 381d029e7a ("fix: serdes: a38x, a39x: Improve USB3 electrical
configuration")

Improves electrical USB3 receiver jitter tolerance test:

- De-Emphasize force, in functional mode the transmitter should always
have 3.5db de-emphasize, so we are forcing it.

- After forcing De-Emphasize, choose 3.5db (After forcing, default is
6dB so need to change it to 3.5dB).

- Align90 set to 0x58 - this is the sample point in the receiver, after
the clock is recovered this sampler samples at the chosen value, usually
it is supposed to be 0x60(which is the center of the eye), but sometimes
after adding jitter and ISI the center of the eye can move slightly and
the sample point is not necessarily the exact center, and after
optimization (searching the middle of the eye manually) it was seen that
the center of the eye is actually 0x58 and not 0x60.

- FFE Res and FFE Cap set to 0xE & 0xF respectively: improves this
settings is adequate according to how the USB3 spec defines the
interconnect, thus improves USB3 jitter tolerance settings.

- Change the resolution of the DFE to 0x3 which is 6mV(highest
resolution) , this avoids the DFE to saturate and cease to work.

- HPF set to 0x3 which is 5Khz high pass filter, the function of the HPF
is to filter the low frequency patterns(below 5Khz) to make sure that
the signal is not a noise, the setting before was 0x1(205Khz), and the
change came since the USB3 CP0 pattern, that is used in the USB3 jitter
tolerance testing, is similar to PRBS15, which has 2^15=32768bits which
is 32768*200ps (200ps is one Unit interval in USB3(5Gbps)) = 6.5us,
which is in frequency terms: 152Khz. since the PRBS15 is a random
pattern and can theoretically have once in a while a pattern that will
be at frequency of 152Khz, hence the previous setting (205khz HPF) can
possibly filter this pattern which can cause to an error in the
receiver, thus this change to avoid such scenarios.

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Signed-off-by: René Straub <rene.straub@netmodule.com>
Reviewed-by: Stefan Roese <sr@denx.de>