mx6: ddr: allow 32 cycles for DQS gating calibration

The DDR calibration code is only setting flag DG_CMP_CYC (DQS gating sample
cycle) for the first PHY.

Set the 32-cycle flag for both PHYs and clear when done so the MPDGCTRL0
output value isn't polluted with calibration artifacts.

Signed-off-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Marek Vasut <marex@denx.de>