am33xx: Provide a function to set up the debug UART

Since commit 0dba4586 ("arm: Init the debug UART") the debug UART is
set up in _main() before early_system_init() is called.

Add a suitable board_debug_uart_init() function to set up the UART in
SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>