AMD/bettong: Add UART support

The function delay in uart8250mem.c is not enough for hudson. I guess
there are some problems in lapic_timer(). I uploaded a patch to gerrit
to show the way to enable UART feature.
http://review.coreboot.org/#/c/12343/4

Currently the HUDSON_UART is unchecked by default. Select HUDSON_UART to
enable this feature.

The UART is test at BIOS stage.

Since it is not a standart UART device, the windows internal UART driver
doesnt support it. I guess we need a driver to use it on windows.

Change-Id: I4cec833cc2ff8069c82886837f7cbd4483ff11bb
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/11749
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/amd/bettong/romstage.c

@@ -39,6 +39,9 @@ void cache_as_ram_main(unsigned long bist, unsigned long cpu_init_detectedx)
 	if (!cpu_init_detectedx && boot_cpu()) {
 		post_code(0x30);
 
+#if IS_ENABLED(CONFIG_HUDSON_UART)
+		configure_hudson_uart();
+#endif
 		post_code(0x31);
 		console_init();
 	}