presence: Un-hardcode input device path

Allow a device path to be configured.

Change-Id: I2eabd71747b124eae96738d1332f58b96ece6dc9
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/presence/example/example.yaml b/presence/example/example.yaml
index 140e1ec..69a322b 100644
--- a/presence/example/example.yaml
+++ b/presence/example/example.yaml
@@ -37,6 +37,7 @@
     - type: gpio
       key: 123
       physpath: /sys/devices/foo/bar
+      devpath: /dev/input/by-path/platform-gpio-keys-event
 
 - name: Example Fan2
   description: >
@@ -58,6 +59,7 @@
     - type: gpio
       key: 124
       physpath: /sys/devices/foo/bar
+      devpath: /dev/input/by-path/platform-gpio-keys-polled-event
     - type: tach
       sensors:
         - fan2
@@ -75,6 +77,7 @@
     - type: gpio
       key: 125
       physpath: /sys/devices/foo/bar
+      devpath: /dev/input/by-path/platform-gpio-keys-polled-event
     - type: tach
       sensors:
         - fan3