ti_omap3_common: Add CONFIG_SYS_NS16550_COMx entries

Several boards do not use the default UART3, so they do a check
for ifdef CONFIG_SPL_BUILD and enable the pointer for
CONFIG_SYS_NS16550_COMx to point to OMAP34XX_UARTx.

Let's consoldate this all into one place, and remove them from the
individual boards.

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