arch: arm: cortex_a_r: update ROM region size calculation

update the directive that calculates the ROM region size
bit mask for the MPU setup.

Signed-off-by: Immo Birnbaum <[email protected]>

Author: ibirnbaum

include/zephyr/arch/arm/cortex_a_r/scripts/linker.ld

@@ -250,7 +250,7 @@ SECTIONS
     __rodata_region_end = .;
     __rom_region_end = .;
     MPU_ALIGN(__rodata_region_end - __rom_region_start);
-    _image_rom_end_order = (LOG2CEIL(__rom_region_end) - 1) << 1;
+    __rom_region_mpu_size_bits = (LOG2CEIL(__rodata_region_end - __rom_region_start) - 1) << 1;
 
     GROUP_END(ROMABLE_REGION)