ata: ahci_tegra: Update initialization sequence

Update the controller initialization sequence and move Tegra124
specifics to tegra124_ahci_init.

Signed-off-by: Preetham Chandru R <pchandru@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>