AArch32: Enable override of plat_set_my_stack/plat_get_my_stack

This patch makes the default MP definitions of plat_get_my_stack()
and plat_set_my_stack() as weak so that they can be overridden by
the AArch32 Secure Payload if it requires.

Change-Id: I3b6ddff5750443a776505e3023ff2934227592b6
Signed-off-by: Soby Mathew <soby.mathew@arm.com>