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/include/gpio.h

@@ -40,6 +40,16 @@ int _gpio_base3_value(gpio_t gpio[], int num_gpio, int binary_first);
  */
 const char *gpio_acpi_path(gpio_t gpio);
 
+/*
+ * This function may be implemented by SoC/board code to provide
+ * a mapping from the internal representation of a GPIO to the 16bit
+ * value used in an ACPI GPIO pin table entry.
+ *
+ * If not implemented by the SOC the default handler will return 0
+ * because the underlying type of gpio_t is unknown.
+ */
+uint16_t gpio_acpi_pin(gpio_t gpio);
+
 /*
  * Read the value presented by the set of GPIOs, when each pin is interpreted
  * as a base-2 digit (LOW = 0, HIGH = 1).