skylake: Add handler for finding ACPI path for GPIO

Add a handler for the Intel Skylake SOC to return the ACPI path for
GPIOs.  Since all GPIOs are handled by the same controller they all
have the same ACPI path and this is a simple handler that just returns
a pointer to the GPIO device that is defined in the DSDT.

Change-Id: I24ff3a6f2479d9e7eeace65d49e2f6c2e070f3e9
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/14843
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/soc/intel/skylake/gpio.c

@@ -398,3 +398,8 @@ void gpio_set(gpio_t gpio_num, int value)
 	write32(&dw_regs[0], reg);
 	/* GPIO port ids support posted write semantics. */
 }
+
+const char *gpio_acpi_path(gpio_t gpio_num)
+{
+	return "\\_SB.PCI0.GPIO";
+}