soc/cavium: Clean uart code

* Only init UART_FOR_CONSOLE
* Allow UART init with zero baudrate.
* Init UART0..3 on Cavium's cn8100_sff_evb to fix kernel panic.

Tested on CN8100_SFF_EVB.

Change-Id: I1043b30318ec6210e2dd6b7ac313a41171d37f55
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25448
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>

src/soc/cavium/cn81xx/uart.c

@@ -232,6 +232,14 @@ int uart_setup(const size_t bus, int baudrate)
 	ctl.s.csclk_en = 1;
 	write64(&uart->uctl_ctl, ctl.u);
 
+	/**
+	 * Exit here if the UART is not going to be used in coreboot.
+	 * The previous initialization steps are sufficient to make the Linux
+	 * kernel not panic.
+	 */
+	if (!baudrate)
+		return 0;
+
 	/**
 	 * 7. Initialize the integer and fractional baud rate divider registers
 	 * UARTIBRD and UARTFBRD as follows: