mb/siemens/mc_ehl2: Set I2C bus 1 speed to 100 kHz

Since the new RTC is located in I2C bus 1 now, set the bus speed to
100 kHz as well.

Change-Id: Ica9468e559bc654545592a9b4d23f3164eafca8a
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67102
Reviewed-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/siemens/mc_ehl/variants/mc_ehl2/devicetree.cb

@@ -109,6 +109,9 @@ chip soc/intel/elkhartlake
 	register "pse_tsn_phy_irq_edge[1]" = "RISING_EDGE"
 
 	register "common_soc_config" = "{
+		.i2c[1] = {
+			.speed = I2C_SPEED_STANDARD,
+		},
 		.i2c[2] = {
 			.speed = I2C_SPEED_STANDARD,
 		},