am33xx: Move the call to ddr_pll_config, make it take the frequency

Depending on if we have DDR2 or DDR3 on the board we will need to call
ddr_pll_config with a different value.  This call can be delayed
slightly to the point where we know which type of memory we have.

Signed-off-by: Tom Rini <trini@ti.com>