drivers: cpuidle: implement OF based idle states infrastructure

drivers: cpuidle: implement OF based idle states infrastructure

On most common ARM systems, the low-power states a CPU can be put into are
not discoverable in HW and require device tree bindings to describe
the respective power domains, power down protocol and idle states parameters.

In order to enable DT based idle states and configure idle drivers, this
patch implements the bulk infrastructure required to parse the device tree
idle states bindings and initialize the corresponding CPUidle driver states
data.

Code that initializes idle states checks the CPU idle driver cpumask so
that multiple CPU idle drivers can be initialized through it in the
kernel. The CPU idle driver cpumask defines which idle states should be
considered valid for the driver, ie idle states that are valid on a set
of cpus the idle driver manages.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>