cleanup: Don't shadow NULL with an enum

For the few platforms without gpio.inc, use this

  enum gpio_signal {
     GPIO_COUNT
  };

instead of this

  enum gpio_signal {
     NULL
  };

The only reason this worked at all is that the headers are
included in a particular order.

BUG=none
BRANCH=none
TEST=make buildall

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Change-Id: I81533f3614d0b4c7389f9edd42cd8ac018581f46
Reviewed-on: https://chromium-review.googlesource.com/278120
Reviewed-by: Randall Spangler <rspangler@chromium.org>

include/gpio.h

@@ -50,7 +50,7 @@
  * define an emtpy one here.*/
 #ifndef __CROS_EC_GPIO_SIGNAL_H
 enum gpio_signal {
-	NULL
+	GPIO_COUNT
 };
 #endif /* __CROS_EC_GPIO_SIGNAL_H */