arm: mvebu: a38x: Configurable USB2 high-speed impedance threshold

Hardware testing of a board using the Armada 385 has shown that an
impedance threshold setting of 0x7 performs better in an eye-diagram
test than with Marvell's recommended value 0x6.

As other boards may still perform better with Marvell's reccomended value,
a configuration option is added with a default value of 0x6.

Signed-off-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Reviewed-by: Stefan Roese <sr@denx.de>