am33xx: Enable DDR3 for DDR3 version of beaglebone

DDR3 support is tested and working with beaglebone hardware. Include a check
for this board type and configure DDR3. The timings and other configuration
match EVM SK.

Signed-off-by: Joel A Fernandes <joelagnel@ti.com>
Acked-by: Jason Kridner <jdk@ti.com>