arm: mx6: ddr: add pd_fast_exit flag to system information

DDR3 has a special Precharge power-down mode: fast-exit vs slow-exit.

In slow-exit mode the DLL is off but in some quiescent state that makes it easy
to turn on again in tXPDLL cycles (about 10tCK) vs the full tDLLK (512tCK).
In fast-exist mode the DLL is maintained such that it is ready again in about
3tCK.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>