soc/amd/stoneyridge: fix gpio_acpi_path()

The path for the GPIO devices needs to be '\_SB', not '\SB'. Fix
the path so that it references the system bus.

BUG=b:72121803

Change-Id: I7c6c38ecea0f8f95ff52b3390f92c5b7e79bcd6d
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/23501
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/stoneyridge/gpio.c

@@ -99,7 +99,7 @@ void gpio_output(gpio_t gpio_num, int value)
 
 const char *gpio_acpi_path(gpio_t gpio)
 {
-	return "\\SB.GPIO";
+	return "\\_SB.GPIO";
 }
 
 uint16_t gpio_acpi_pin(gpio_t gpio)