[nrfconnect] Fixed MPU fault during DFU over SMP (#19227)

Flash NOP device was disabled during the last
nRF Connect SDK version update, but it turn out to cause
MPU fault during DFU over SMP.

Restored enabling flash NOP device.

config/nrfconnect/chip-module/Kconfig.features

@@ -93,6 +93,12 @@ config SYSTEM_WORKQUEUE_STACK_SIZE
 
 if SOC_SERIES_NRF53X
 
+# FLASH nop device is enabled to prevent bus faults when mcumgr tries to access
+# simulated partition with network core image data.
+config FLASH_NOP_DEVICE
+	bool
+	default y
+
 # Enable custom SMP request to erase settings partition.
 config MCUMGR_GRP_ZEPHYR_BASIC
 	bool