mb/clevo/l140cu: work around PECI staying high when idle, blocking s0ix

According to Intel doc# 575683 the PECI bus should be low when idle and
is pulled up by clients with strong drive. However, for unknown reasons
the bus stays high on this board, blocking s0ix entry.

The PECI reference schematic in the ASPEED AST2400 BMC datasheet
(actually not related to this board) says that a pull-down is *required*
for the idle state.

This might be just a requirement of this BMC, since this is nowhere
documented in Intel datasheets, schematics or elsewhere. However,
configuring a weak pull-down (20 k) on the PECI pad indeed solves this
problem for now.

Change-Id: Ib5a6b0ad3553c2cf795037d6a1982102bcb04644
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68793
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/clevo/cml-u/variants/l140cu/gpio.c

@@ -233,6 +233,9 @@ static const struct pad_config gpio_table[] = {
 	PAD_NC(GPP_H21, NONE),
 	PAD_NC(GPP_H22, UP_20K),
 	PAD_NC(GPP_H23, UP_20K),
+
+	/* ------- GPIO Group CPU ------- */
+	PAD_CFG_NF(PECI_IO, DN_20K, DEEP, NF1),		/* PECI */
 };
 
 void variant_configure_gpios(void)