ARM: clps711x: Add cpuidle driver

Add cpuidle support for ARM Cirrus Logic CLPS711X CPUs.
This CPU has an unique internal register and write to this location
will put the system into the Idle State by halting the clock to the
processor until an interrupt is generated.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>