arm: Add weak function definition for random pool intialization

The random pool relies on devices and other items in the system
to add entropy to the pool. Most of these devices may not be
added until later in the bootup process. This leaves a large
period of time where the random pool may not actually give
random numbers. Add a weak function for devices to override
with their own function to setup the random pool.

Change-Id: I0de63420b11f1dd363ccd0ef6ac0fa4a617a1152
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>