ChromeOS: Promote variant_cros_gpio()

The only purpose of mainboard_chromeos_acpi_generate()
was to pass cros_gpio array for ACPI \\OIPG package
generation.

Promote variant_cros_gpio() from baseboards to ChromeOS
declaration.

Change-Id: I5c2ac1dcea35f1f00dea401528404bc6ca0ab53c
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58897
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/mainboard/google/butterfly/chromeos.c

@@ -53,9 +53,10 @@ static const struct cros_gpio cros_gpios[] = {
 	CROS_GPIO_WP_AL(WP_GPIO, CROS_GPIO_DEVICE_NAME),
 };
 
-void mainboard_chromeos_acpi_generate(void)
+const struct cros_gpio *variant_cros_gpios(size_t *num)
 {
-	chromeos_acpi_gpio_generate(cros_gpios, ARRAY_SIZE(cros_gpios));
+	*num = ARRAY_SIZE(cros_gpios);
+	return cros_gpios;
 }
 
 int get_ec_is_trusted(void)