tegra21: emc: Only poll single rank for PD

tegra21: emc: Only poll single rank for PD

Only poll the single active rank for power-down status when checking if
the DRAM has left auto power-down state and there's only a single rank
of DRAM.

Bug 1906919

Change-Id: I5bc8c926e03268785425b337780e5720414342de
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/1471775
(cherry picked from commit fdb38b913a0eda8dbb545528192e542fcdd37199)
Reviewed-on: https://git-master.nvidia.com/r/1934345
(cherry picked from commit 6ee8c4091545e7917e8cde7e0f37849517a3a30e)
Reviewed-on: https://git-master.nvidia.com/r/1935485
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>