feat(cpu): add library support for Poseidon CPU

This patch adds the basic CPU library code to support the Poseidon CPU
in TF-A. Poseidon is derived from HunterELP core, an implementation of
v9.2 architecture. Currently, Hunter CPU the predecessor to HunterELP,
is supported in TF-A. Accordingly the Hunter CPU library code has been
as the base and adapted here.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I406b4de156a67132e6a5523370115aaac933f18d