[nrf noup] sample/**/smp_svr: nrf54l15pdk external memory dts overlay

Added overlay with m25r64 on-DK memory selecton.
This aim to support testing DFU with secondary partition moved to the
external flash.

Signed-off-by: Andrzej Puzdrowski <[email protected]>
(cherry picked from commit ae151cd)
(cherry picked from commit 36e84c5)

Author: nvlsianpu

samples/subsys/mgmt/mcumgr/smp_svr/boards/nrf54l15pdk_nrf54l15_cpuapp_ext_flash.overlay

@@ -0,0 +1,9 @@
+/ {
+	chosen {
+		nordic,pm-ext-flash = &mx25r64;
+	};
+};
+
+&mx25r64 {
+	status = "okay";
+};