gpio: cosmetic changes to tristate_gpios.c

This patch makes a few cosmetic changes:
- Rename tristate_gpios.c to gpio.c since it will soon be used for
  binary GPIOs as well.
- Rename gpio_get_tristates() to gpio_base3_value() - The binary
  version will be called gpio_base2_value().
- Updates call sites.
- Change the variable name "id" to something more generic.

BUG=none
BRANCH=none
TEST=compiled for veyron_pinky and storm

Change-Id: I36d88c67cb118efd1730278691dc3e4ecb6055ee
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/228324

src/include/gpio.h

@@ -37,12 +37,12 @@ void gpio_output(gpio_t gpio, int value);
 /*
  * Read the value presented by the set of GPIOs, when each pin is interpreted
  * as a base-3 digit (LOW = 0, HIGH = 1, Z/floating = 2).
- * Example: X1 = Z, X2 = 1 -> gpio_get_tristates({GPIO(X1), GPIO(X2)}) = 5
+ * Example: X1 = Z, X2 = 1 -> gpio_base3_value({GPIO(X1), GPIO(X2)}) = 5
  * BASE3() from <base3.h> can generate numbers to compare the result to.
  *
  * gpio[]: pin positions to read. gpio[0] is less significant than gpio[1].
  * num_gpio: number of pins to read.
  */
-int gpio_get_tristates(gpio_t gpio[], int num_gpio);
+int gpio_base3_value(gpio_t gpio[], int num_gpio);
 
 #endif /* __SRC_INCLUDE_GPIO_H__ */