add GPIO definition

Add the enum definition we can rely on for the next patches.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
diff --git a/display.c b/display.c
index ebc4de6..f06387c 100644
--- a/display.c
+++ b/display.c
@@ -63,6 +63,7 @@
 	[CLOCK]     = { .name = "Clocks"     },
 	[REGULATOR] = { .name = "Regulators" },
 	[SENSOR]    = { .name = "Sensors"    },
+	[GPIO]      = { .name = "Gpio"    },
 };
 
 static void display_fini(void)
diff --git a/display.h b/display.h
index 9e4e21a..035b2e9 100644
--- a/display.h
+++ b/display.h
@@ -13,7 +13,7 @@
  *       - initial API and implementation
  *******************************************************************************/
 
-enum { CLOCK, REGULATOR, SENSOR };
+enum { CLOCK, REGULATOR, SENSOR, GPIO };
 
 struct display_ops {
 	int (*display)(bool refresh);