am33xx: support board specific ddr settings

Move the hardcoded ddr2/ddr3 settings for the ti boards to board code,
so other boards can use different types/timings.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>
[trini: Make apply with rtc32k_enable() in the file]
Signed-off-by: Tom Rini <trini@ti.com>