ARM: am3517_evm: Enable SPL_OF_CONTROL and SPL_OF_PLATDATA

The SPL doesn't have much room, so in order to support OF_CONTROL
in SPL, we need the extra functionality of SPL_OF_PLATDATA.

Adding these features allows us to remove a small part of code without
losing the serial port during SPL.

Signed-off-by: Adam Ford <aford173@gmail.com>