arm: tegra: power: add reference counters for pcie

arm: tegra: power: add reference counters for pcie

Due to HW Bug 1320346, both XUSB and PCIE modules need to control powergating
on PCIE block.  Implement reference counter mechanism so that powergating is
done only when neither of these two drivers are active.

Bug 1451279
Bug 1455032
Bug 200027067

Change-Id: I6866ba0fb47433e211360b722abbb1b2c1a05c35
Reviewed-on: http://git-master/r/418060
(cherry picked from commit 70be55b824632252be36f6ee0d495d4c0f4e507c)
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Reviewed-on: http://git-master/r/498659
Reviewed-by: Matthew Pedro <mapedro@nvidia.com>
Tested-by: Matthew Pedro <mapedro@nvidia.com>