mb/google/peppy: add _DSD to touchscreen ACPI

Recent changes to the Atmel touchscreen driver in the mainline
kernel broke functionality with devices running upstream coreboot,
due relying on another driver (chromeos_laptop) which makes the
assumption that the i2c devices are be in PCI mode (as with the
stock Google firmware) rather than in ACPI mode as they are in
upstream coreboot.

Mitigate this by adding the required devicetree property so the
Atmel toushcreen driver will correctly attach without the use
of chromeos_laptop.

Test: build/boot peppy on 4.18+ kernel, verify touchscreen working

Change-Id: I05df8367886eef55b409590f75a68d98d4e5fbdf
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37915
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicolò
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/google/slippy/variants/peppy/include/variant/acpi/mainboard.asl

@@ -140,6 +140,13 @@ Scope (\_SB.PCI0.I2C1)
 			}
 		}
 
+		Name (_DSD, Package () {
+			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+			Package () {
+				Package () {"compatible", "atmel,maxtouch"},
+			}
+		})
+
 		Method (_STA)
 		{
 			If (LEqual (\S2EN, 1)) {