mb/siemens/mc_ehl3: Enable PWM passthrough mode on PTN3460

The connected panel on this mainboard gets the PWM frequency directly
from the Elkhart Lake CPU. The PWM controls the brightness of the
backlight. Therefore, it is necessary to activate the PWM passthrough
mode in the PTN3460 eDP-to-LVDS bridge (see PTN3460 Programming Guide -
5. Configuration Registers).

Link to PTN3460 Programming Guide:
https://web.archive.org/web/20230908074244/https://www.nxp.com/docs/en/application-note/AN11128.pdf

BUG=none
TEST=Boot into Linux and change the brightness of the screen

Change-Id: Ia0a329426e585b6243c8888806befbe4f6ec2998
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77856
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jan Samek <jan.samek@siemens.com>

src/mainboard/siemens/mc_ehl/variants/mc_ehl3/lcd_panel.c

@@ -90,6 +90,8 @@ enum cb_err mainboard_ptn3460_config(struct ptn_3460_config *cfg)
 	cfg->t5_delay = 0x01;
 	/* Enable backlight control. */
 	cfg->backlight_ctrl = 0x00;
+	/* Enable PWMI passthrough mode. */
+	cfg->pin_cfg_ctrl1 = 0x10;
 
 	return CB_SUCCESS;
 }