target/arm: cpu: mark WHPX as supporting PSCI 1.3

Hyper-V supports PSCI 1.3, and that implementation is exposed through
WHPX.

Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>