drivers: cpuidle: initialize big.LITTLE driver through DT

With the introduction of DT based idle states, CPUidle drivers for ARM
can now initialize idle states data through properties in the device tree.

This patch adds code to the big.LITTLE CPUidle driver to dynamically
initialize idle states data through the updated device tree source file.

Cc: Chander Kashyap <k.chander@samsung.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>