This patch adds the PSCI library integration guide for AArch32 ARMv8-A
systems `psci-lib-integration-guide.md` to the documentation. The
patch also adds appropriate reference to the new document in
the `firmware-design.md` document.
Change-Id: I2d5b5c6b612452371713399702e318e3c73a8ee0
Soby Mathew