lynxpoint: Add interrupt for GPIO controller in ACPI device

The GPIO controller uses IRQ14 as an active high level triggered
source for GPIOs that are configured to trigger shared interrupt.

BUG=chrome-os-partner:22996
BRANCH=samus
TEST=emerge-samus chromeos-coreboot-samus

This was also tested on bolt by configuring the touchscreen to use
a shared GPIO interrupt:

localhost ~ $ grep atmel_mxt_ts /proc/interrupts
54:    24    188    93    124    LP-GPIO-demux    atmel_mxt_ts

Change-Id: I3765120112bae11407e5b2020399d0d0b8e3cef8
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/171901
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/src/southbridge/intel/lynxpoint/acpi/serialio.asl b/src/southbridge/intel/lynxpoint/acpi/serialio.asl
index 4c0d36b..3e68c13 100644
--- a/src/southbridge/intel/lynxpoint/acpi/serialio.asl
+++ b/src/southbridge/intel/lynxpoint/acpi/serialio.asl
@@ -502,6 +502,8 @@
 			,            // ResourceSourceIndex
 			,            // ResourceSource
 			BAR0)
+		Interrupt (ResourceConsumer,
+			Level, ActiveHigh, Shared, , , ) {14}
 	})
 
 	Method (_CRS, 0, NotSerialized)