[nrf fromtree] drivers: firmware: nrf_ironside: Guard behind CONFIG_NRF_IRONSIDE

Add a promptless symbol that can be selected by different drivers. It is
there to ensure that the corresponding library is not added to the build
unless it contains at least one source file.

Signed-off-by: Grzegorz Swiderski <[email protected]>
(cherry picked from commit 3353bbc)

Author: 57300

drivers/firmware/CMakeLists.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
 # zephyr-keep-sorted-start
-add_subdirectory(nrf_ironside)
 add_subdirectory_ifdef(CONFIG_ARM_SCMI scmi)
+add_subdirectory_ifdef(CONFIG_NRF_IRONSIDE nrf_ironside)
 # zephyr-keep-sorted-stop

drivers/firmware/nrf_ironside/Kconfig

@@ -1,10 +1,16 @@
 # Copyright (c) 2025 Nordic Semiconductor ASA
 # SPDX-License-Identifier: Apache-2.0
 
+config NRF_IRONSIDE
+	bool
+	depends on SOC_NRF54H20_IRON
+	help
+	  This is selected by drivers interacting with Nordic IRONside firmware.
+
 config NRF_IRONSIDE_CALL
 	bool
 	depends on DT_HAS_NORDIC_IRONSIDE_CALL_ENABLED
-	depends on SOC_NRF54H20_IRON
+	select NRF_IRONSIDE
 	select EVENTS
 	select MBOX
 	help