ARM: cpuidle: Add a cpuidle ops structure to be used for DT

ARM: cpuidle: Add a cpuidle ops structure to be used for DT

The current state of the different cpuidle drivers is the different PM
operations are passed via the platform_data using the platform driver
paradigm.

This approach allowed to split the low level PM code from the arch specific
and the generic cpuidle code.

Unfortunately there are complaints about this approach as, in the context of the
single kernel image, we have multiple drivers loaded in memory for nothing and
the platform driver is not adequate for cpuidle.

This patch provides a common interface via cpuidle ops for all new cpuidle
driver and a definition for the device tree.

It will allow with the next patches to a have a common definition with ARM64
and share the same cpuidle driver.

The code is optimized to use the __init section intensively in order to reduce
the memory footprint after the driver is initialized and unify the function
names with ARM64.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Rob Herring <robherring2@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>