ata: ahci_tegra: Change include to fix compilation

Before this patch, the driver included <linux/tegra-powergate.h>,
which was effectively renamed to <soc/tegra/pmc.h> at about the same
time the ahci_tegra series landed. Fix the include path so that the
driver compiles.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>