boards: arm: mps2_an521: Resize _remote and _ns

The `mps2_an521_remote` and `mps2_an521_ns` targets have the same
memory layout for code and ram, meaning that you can't use TF-M
(`mps2_an521_ns`) and the second core (`mps2_an521_remote`) at the
same time.

This commit updates the memory map of the `_ns` build targets as
follows:

- Reduces the code memory region from 1 MB to 512 KB, maintaining the
  existing base memory address of `0x0010 0000`
- Maintains the existing 512 KB ram memory at `0x2810 0000`

It updates the `_remote` target as follows:

- Reserves 468 KB code memory at address `0x0038 B000`
- Reserves 512 KB ram memory at address `0x2818 0000`

This ensures that the code region for the `_remote` target doesn't
overlap with the code region used by the single flash image layout
defined upstream in TF-M, which the `_ns` target is based upon.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>