arm: do not force d-cache enable on all boards

c2dd0d45 added dcache_enable()
to board_init_r(). This enables d-cache for all ARM boards.
As a result some of the arm boards that are not cache-ready
are broken. Revert this change and allow platform code to
take the decision on d-cache enabling.

Also add some documentation for cache usage in ARM.

Signed-off-by: Aneesh V <aneesh@ti.com>