[nrf fromlist] samples: drivers: mbox: Disable UART Flow Control on nrf54h20 and nrf54l15

Nrf54h20dk and nrf54l15 have UART Flow Control enabled
by default on all cores.
It's a source of problem when mbox sample is run in Twister.
Twister opens serial port only on host core,
thus nobody confirms reception of logs from remote core.
As a result, remote core stucks on printing boot banner.

Twister fails sample due to the timeout while waiting for messages
from remote to host.

Disable Flow Control on remote core when mbox sample is executed
on nrf54h20 or nrf54l15.

Upstream PR: zephyrproject-rtos/zephyr#75309

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

samples/drivers/mbox/remote/boards/nrf54l15pdk_nrf54l15_cpuflpr.overlay

@@ -18,3 +18,7 @@
 &cpuflpr_vevif_tx {
 	status = "okay";
 };
+
+&uart30 {
+	/delete-property/ hw-flow-control;
+};

samples/drivers/mbox/remote/boards/nrf54l15pdk_nrf54l15_cpuflpr_xip.overlay

@@ -18,3 +18,7 @@
 &cpuflpr_vevif_tx {
 	status = "okay";
 };
+
+&uart30 {
+	/delete-property/ hw-flow-control;
+};