purism/librem_cnl: Add Kconfig to enable Mini UART

Librem Mini has a UART (accessible with soldering), which is very
useful for work on coreboot.  It can be used for coreboot/SeaBIOS/Linux
boot logs, or as a general purpose UART.

Change-Id: I38ad5f19da6af5ed286ad3583f34b824a3660916
Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68551
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/purism/librem_cnl/Kconfig

@@ -60,3 +60,19 @@ config NO_POST
 	default y
 
 endif
+
+config ENABLE_EC_UART1
+	bool "Enable EC UART1"
+	depends on BOARD_PURISM_LIBREM_MINI || BOARD_PURISM_LIBREM_MINI_V2
+	default n
+	select DRIVERS_UART_8250IO
+	help
+	  Enable UART1 on the EC.
+
+	  This UART can be used for boot logging by coreboot, SeaBIOS, or
+	  Linux.  It also works as a general-purpose UART.
+
+	  Soldering is required to access these signals.  Locate the pads for
+	  U81 on the bottom of the board near the front edge; the IC is not
+	  populated.  TX is pin 14, RX is pin 19.  The signals are 3.3V (do NOT
+	  connect directly to an RS-232 serial port).

src/mainboard/purism/librem_cnl/variants/librem_mini/Makefile.inc

@@ -2,3 +2,4 @@
 
 all-y += die.c
 smm-y += die.c
+bootblock-$(CONFIG_ENABLE_EC_UART1) += com_init.c

src/mainboard/purism/librem_cnl/variants/librem_mini/com_init.c

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <bootblock_common.h>
+#include <device/pnp_ops.h>
+
+#define SERIAL_DEV PNP_DEV(0x6E, 1) /* ITE8528 UART1 */
+
+void bootblock_mainboard_early_init(void)
+{
+	/* Enable the serial port inside the EC */
+	pnp_set_logical_device(SERIAL_DEV);
+	pnp_set_enable(SERIAL_DEV, 1);
+}