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.

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>
(cherry picked from commit 63a0c80ce5)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6541
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/southbridge/intel/lynxpoint/acpi/serialio.asl

@@ -546,6 +546,8 @@ Device (GPIO)
 			,            // ResourceSourceIndex
 			,            // ResourceSource
 			BAR0)
+		Interrupt (ResourceConsumer,
+			Level, ActiveHigh, Shared, , , ) {14}
 	})
 
 	Method (_CRS, 0, NotSerialized)