Helper macro, MPU_ALIGN() is used by script
gen_app_partitions.py, so the macro needs to be available,
if the APP Shared memory feature is to be used. This commit
defines MPU_ALIGN() in the ARC linker.ld script.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>