context_mgmt: Make cm_init_context_common public

context_mgmt: Make cm_init_context_common public

This function can be currently accessed through the wrappers
cm_init_context_by_index() and cm_init_my_context(). However, they only
work on contexts that are associated to a CPU.

By making this function public, it is possible to set up a context that
isn't associated to any CPU. For consistency, it has been renamed to
cm_setup_context().

Change-Id: Ib2146105abc8137bab08745a8adb30ca2c4cedf4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>