drivers: arm: cpuidle: support ARMv7 targets for lpm governor

QCOM's low power mode cpuidle driver support ARM v8 targets primarily.
The initialization of PSCI for ARM v7 targets is done through the
generic ARM cpuidle driver. Since LPM driver replaces the ARM's default
driver for QCOM SoC's including the ARM v7 variants, we now have the
responsibility of initializing the PSCI cpuidle ops.

Also, since QCOM SoCs do not use ARM idle state definitions as suggested
in [1], the initialization of PSCI cpuidle ops would fail. Make the PSCI
cpuidle ops initialization conditional.

[1]. Documentation/devicetree/bindings/arm/idle-states.txt

Change-Id: Ifef6c5bdd76e93d34af45b51efbe12f784421746
Signed-off-by: Lina Iyer <ilina@codeaurora.org>