zynqmp: Add simple implementation of zynqmp_validate_power_state()

Implementation is based on arm_validate_power_state().
This function is called during CPU_SUSPEND PSCI call to validate
power_state parameter. If state is valid this function populate it
in req_state array as power domain level specific local state.
ATF platform migration guide chapter 2.2 defines this function as
mandatory for PSCIv1.0 CPU_SUSPEND support.

Signed-off-by: Stefan Krsmanovic <stefan.krsmanovic@aggios.com>