drivers/uart/pl011.c Add read support

Implement uart_rx_byte

Change-Id: I5a33b3cd4b333fb244bc55747014a301021e25e0
Signed-off-by: Marcello Sylvester Bauer <info@marcellobauer.com>
Reviewed-on: https://review.coreboot.org/25623
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/uart/pl011.c

@@ -42,7 +42,11 @@ void uart_tx_flush(int idx)
 
 unsigned char uart_rx_byte(int idx)
 {
-	return 0;
+	struct pl011_uart *regs = uart_platform_baseptr(idx);
+
+	while (read32(&regs->fr) & PL011_UARTFR_RXFE)
+		;
+	return read8(&regs->dr);
 }
 
 #ifndef __PRE_RAM__