Instead of allocating 'n-1' states in psci_power_state to manage 'n'
idle states which include "ARM WFI" state, it would be simpler to have
1:1 mapping between psci_power_state and cpuidle driver states.

ARM WFI state(i.e. idx == 0) is handled specially in the generic macro
CPU_PM_CPU_IDLE_ENTER_PARAM and hence state[-1] is not possible. However
for sake of code readability, it is better to have 1:1 mapping and not
use [idx - 1] to access psci_power_state corresponding to driver cpuidle
state for idx.

psci_power_state[0] is default initialised to 0 and is never accessed
while entering WFI state.
Reported-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>