am33xx: Pass to config_ddr the type of memory that is connected

We need to pass in the type of memory that is connected to the board.
The only reliable way to do this is to know what type of board we are
running on (which later will be knowable in s_init()).  For now, pass in
the value of DDR2.

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