arm: mx6: ddr: configure MMDC for slow_pd

arm: mx6: ddr: configure MMDC for slow_pd

According to MX6 TRM, both MMDC and DRAM should be configured to
the same powerdown precharge. Currently, mx6_dram_cfg()
configures MMDC for fast pd (MDPDC[7] = 0), and the DRAM for
'slow exit (DLL off)' (MR0[12] = 0).

Configure MMDC for slow pd.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Tim Harvey <tharvey@gateworks.com>