driver/uart: Introduce a way for mainboard to override the baudrate

The rationale is to allow the mainboard to override the default
baudrate for instance by sampling GPIOs at boot.

A new configuration option is available for mainboards to select
this behaviour. It will then have to define the function
get_uart_baudrate to return the computed baudrate.

Change-Id: I970ee788bf90b9e1a8c6ccdc5eee8029d9af0ecc
Signed-off-by: Julien Viard de Galbert <jviarddegalbert@online.net>
Reviewed-on: https://review.coreboot.org/23713
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/console/Kconfig

@@ -76,6 +76,14 @@ depends on UART_FOR_CONSOLE = 2
 comment "Serial port base address = 0x2e8"
 depends on UART_FOR_CONSOLE = 3
 
+config UART_OVERRIDE_BAUDRATE
+	boolean
+	help
+	  Set to "y" when the platform overrides the baudrate by providing
+	  a get_uart_baudrate routine.
+
+if !UART_OVERRIDE_BAUDRATE
+
 choice
 	prompt "Baud rate"
 	default CONSOLE_SERIAL_115200
@@ -129,6 +137,8 @@ config TTYS0_BAUD
 	help
 	  Map the Baud rates to an integer.
 
+endif
+
 # TODO: Allow user-friendly selection of settings other than 8n1.
 config TTYS0_LCS
 	int