reef: tune trackpad i2c frequency to 400kHz

This brings the frequency down to 400kHz which is spec for
fast i2c.

BUG=chrome-os-partner:58889

Change-Id: Ibc5f152e55ed618f18ac6425264f086b1f2d1ffa
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17215
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/google/reef/variants/baseboard/devicetree.cb

@@ -84,6 +84,17 @@ chip soc/intel/apollolake
 		}
 	}"
 
+	# Limit trackpad speed to 400kHz with manually tuned values.
+	register "i2c[4]" = "{
+		.speed = I2C_SPEED_FAST,
+		.speed_config[0] = {
+			.speed = I2C_SPEED_FAST,
+			.scl_lcnt = 0xd0,
+			.scl_hcnt = 0x68,
+			.sda_hold = 0x27,
+		}
+	}"
+
 	# Minimum SLP S3 assertion width 28ms.
 	register "slp_s3_assertion_width_usecs" = "28000"