arm64: psci: fix cpu_suspend to check idle state type for index

This fix rectifies the psci cpu_suspend implementation to check the
PSCI power state parameter type field associated with the requested idle
state index.

Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>