soc/amd/picasso/uart: factor out console-related functions

Move uart_platform_base and uart_platform_refclk to their own
compilation unit to avoid preprocessor usage. The newly created
compilation unit is only added to the build when PICASSO_CONSOLE_UART
is selected.

Change-Id: I56911addc8c000a0772156e5166720867cdd26fe
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42517
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/southbridge.c

@@ -21,6 +21,7 @@
 #include <soc/i2c.h>
 #include <soc/southbridge.h>
 #include <soc/smi.h>
+#include <soc/uart.h>
 #include <soc/amd_pci_int_defs.h>
 #include <delay.h>
 #include <soc/pci_devs.h>