arm: support very early callback into board functions

Due to changes of the order of initialization of
the ARM-specfic memory management code in 3.0, it
is necessary to create a way to call platform specific
code earlier than is currently possible.

Change-Id: I77dae10c85085358f7240889a50b78b07d5af6d1
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>