arch: arm: Move ARM code to AArch32 sub-directory

Before introducing the code for ARM64 (AArch64) we need to relocate the
current ARM code to a new AArch32 sub-directory. For now we can assume
that no code is shared between ARM and ARM64.

There are no functional changes. The code is moved to the new location
and the file paths are fixed to reflect this change.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>