pistachio: Use passive windowing as DQS gating scheme

Switching from active windowing DQS gating scheme to
passive windowing mode resolves boot stability issues
on chips found to have memory corruption issues during
boot or memory tests.

It was tested on Pistachio bring up board where DDR is
initialized properly and ramstage executed correctly;
We have cycled units over 12,000 times with no boot errors.

This option was chosen over the alternative of using
passive windowing mode for DQS training and after switching
back to active mode, as this option was recommended by
Synopsys. Using the alternative would give different
timing values during training that were not longer accurate
during normal activity.

Change-Id: Ie604eddc0a9a982b2f89198f44deb88a01b7b322
Signed-off-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
Reviewed-on: http://review.coreboot.org/10528
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

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