arm: mvebu: a38x: Adjust UTMI PHY parameters

arm: mvebu: a38x: Adjust UTMI PHY parameters

When running USB compliance tests on our Armada-385 hardware platforms
we have seen some eye mask violations. Marvell's internal documentation
says: Based on silicon test results, it is recommended to change the
impedance calibration threshold setting to 0x6 prior to calibration.

Port changes from Marvell's u-boot fork[1] to address this.

[1] - https://github.com/MarvellEmbeddedProcessors/u-boot-marvell/commit/a6221551

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>