ahci: add support for IBM Akebono platform device

The new IBM Akebono board has a PPC476GTR SoC with an AHCI compliant
SATA controller. This patch adds a compatible property for the new SoC
to the AHCI platform driver.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org