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>