You will have to use direct I/O access. It's not different than doing it from a computer: http://hackspire.unsads.com/wiki/index.php/Memory-mapped_I/O_ports#90020000_-_Serial_UART

Here is a code snippet that was once used in Ndless, equivalent to puts():

void ut_puts(const char *str) {
volatile unsigned *line_status_reg = (unsigned*)0x90020014;
volatile unsigned *xmit_holding_reg = (unsigned*)0x90020000;
while(*str) {
while(!(*line_status_reg & 0b100000)); // wait for empty xmit holding reg
*xmit_holding_reg = *str++;
}
}