ARM: cpuidle-imx*: Report the entered state index

ARM: cpuidle-imx*: Report the entered state index

When a cpuidle driver is called it can choose to enter a different state
that what was asked from above. When this happens it should return the
actual entered_state index for proper accounting.

This fixes the various imx cpuidle drivers which depend on low busfreq
for LOW-POWER-IDLE to correctly report that they entered WAIT instead.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>