board: ti: dra7xx: add mux data for UART3

board: ti: dra7xx: add mux data for UART3

J6 EVM can be built with UART3 as console, but currently
there's nothing muxing UART3 correctly. Likely this only
works because, based on commit log, author was only testing
with UART3 boot and - I assume - ROM code leave UART3 correctly
muxed in that case.

If we want to boot from MMC and still use UART3 as console,
then we need to mux those signals correctly.

Signed-off-by: Felipe Balbi <balbi@ti.com>