linker: Fix missing DEVNULL_REGION for XIP configuration

DEVNULL_REGION was created only for !CONFIG_XIP. Moved definition
outside of that #ifdef.
Removed condition which was repeated as it is present at the beginning
of the file.

(cherry picked from commit 67676d8)

Original-Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
GitOrigin-RevId: 67676d8
Change-Id: I8a574a332ce1f8a1c26708d60fc572e1d7677d0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/4993824
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>

include/zephyr/linker/linker-devnull.h

@@ -65,14 +65,10 @@
 #error "Cannot place devnull segment adjacent to ROM region."
 #endif
 
-#if defined(CONFIG_LINKER_DEVNULL_MEMORY)
-#define DEVNULL_REGION DEVNULL_ROM
-#else
-#define DEVNULL_REGION ROMABLE_REGION
-#endif
-
 #endif /* CONFIG_XIP */
 
+#define DEVNULL_REGION DEVNULL_ROM
+
 #endif /* CONFIG_LINKER_DEVNULL_MEMORY */
 
 #endif /* ZEPHYR_INCLUDE_LINKER_LINKER_DEVNULL_H_ */