boards: use zephyr_i2c instead of stemmma_qt_i2c

Change the two current instances of stemma_qt_i2c to zephyr_i2c, using
an unified label allows using this in shields.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

boards/pimoroni/pico_plus2/pico_plus2.dtsi

@@ -175,4 +175,4 @@ pico_spi: &spi0 {};
 pico_i2c0: &i2c0 {};
 pico_i2c1: &i2c1 {};
 pico_serial: &uart0 {};
-stemma_qt_i2c: &i2c0 {};
+zephyr_i2c: &i2c0 {};

boards/shields/adafruit_aw9523/adafruit_aw9523.overlay

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-&stemma_qt_i2c {
+&zephyr_i2c {
 	status = "okay";
 
 	adafruit_aw9523: aw9523b@58 {