[ARM] Add support for PSCI v0.2 emulation

Currently, we only emulated PSCI v0.1 for Guest VCPUs.

This patch extends current PSCI emulation to emulate both
PSCI v0.1 and PSCI v0.2 for Guest VCPUs.

The PSCI version to emulate will be selected via Guest
device tree node attribute "psci_version".

Signed-off-by: Anup Patel <anup@brainfault.org>