[nrf fromtree] tests: drivers: uart: uart_async_api: Use 4Mbaud for nrf54h20dk uart120

Use high baudrate when testing uart120 on nrf54h20dk/nrf54h20/cpuapp.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit fe6a4d0)

Author: nordic-krch

tests/drivers/uart/uart_async_api/boards/nrf54h20dk_nrf54h20_common.dtsi

@@ -50,6 +50,6 @@ dut2: &uart120 {
 	pinctrl-0 = <&uart120_default_alt>;
 	pinctrl-1 = <&uart120_sleep_alt>;
 	pinctrl-names = "default", "sleep";
-	current-speed = <115200>;
+	current-speed = <4000000>;
 	zephyr,pm-device-runtime-auto;
 };