ARC: init debug uart in early common arc code

The debug UART is intended for use very early in U-Boot to debug
problems before serial drivers are up.

Call debug_uart_init right before board_init_f.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>