[nrf noup] samples/../smp_svr: fix nrf54l15pdk ext flash dts overlay

Align to changes in DTS:
  renamed: rram0 -> cpuapp_rram
  sized up cpauapp_rram region szie as part of it was reserved for
  cpuflpr_rram (which is not used by this config).

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

Author: nvlsianpu

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

@@ -13,7 +13,8 @@
 
 /delete-node/ &storage_partition;
 
-&rram0 {
+&cpuapp_rram {
+	reg = < 0x0 DT_SIZE_K(1524) >;
 	partitions {
 		boot_partition: partition@0 {
 			label = "mcuboot";