sata, highbank: set tx_atten override bits

Some board designs do not drive the SATA transmit lines within the
specification. The ECME can provide override settings, on a per board
basis, to bring the transmit lines within spec. Read those settings
from the DTB and program them in.

At the time of submission, no production hardware requires this patch.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Signed-off-by: Tejun Heo <tj@kernel.org>