highbank: add custom ahci_link_up function

The Calxeda highbank SOC needs a custom sequence to bring up SATA links,
so override ahci_link_up with custom function to handle combophy setup.

Signed-off-by: Mark Langsdorf <mark.langsdorf@gmail.com>
Signed-off-by: Richard Gibbs
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andre Przywara <osp@andrep.de>