subsys: app_memory: Fixed the size calculation for power of 2 MPUs

The size calculation for power of 2 MPUs were incorrect.
The calculation was not taking into account the amount of padding
the linker does when doing the required alignment. Hence the size
being calculated was completely incorrect.

With this patch the code now is optimized and the size of
partitions is now provided by the linker.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>