gpio: Add support for translating gpio_t into ACPI pin

Add a function for an SOC to define that will allow it to map the
SOC-specific gpio_t value into an appropriate ACPI pin.  The exact
behavior depends on the GPIO implementation in the SOC, but it can
be used to provide a pin number that is relative to the community or
bank that a GPIO resides in.

Change-Id: Icb97ccf7d6a9034877614d49166bc9e4fe659bcf
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/15512
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/lib/gpio.c

@@ -145,3 +145,9 @@ __attribute__((weak)) const char *gpio_acpi_path(gpio_t gpio)
 {
 	return NULL;
 }
+
+/* Default handler returns 0 because type of gpio_t is unknown */
+__attribute__((weak)) uint16_t gpio_acpi_pin(gpio_t gpio)
+{
+	return 0;
+}