ARM: PSCI: Support PSCI v0.2

Enhance the PSCI DT editing code to allow setting a PSCI v0.2 compatible
value in the DT. The CONFIG_ option is added to the whitelist to match the
existing PSCI_1_0 option. While not adding new options to Kconfig isn't
ideal, I figure it's better to keep related options together.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>