Define gpio polarity values in one place.

No need to define these everywhere, one place will serve all uses.

BUG=none
TEST=compiled various targets without any problem

Change-Id: Iabf31baad6049c758e078727ba3ebe830c3c7684
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/210921
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/boot/coreboot_tables.h

@@ -197,6 +197,10 @@ struct lb_framebuffer {
 
 #define LB_TAG_GPIO	0x0013
 
+/* Values for the polarity field below. */
+#define ACTIVE_LOW     0
+#define ACTIVE_HIGH    1
+
 struct lb_gpio {
 	uint32_t port;
 	uint32_t polarity;