imx: mx7: psci: support CPU0 on/off

imx: mx7: psci: support CPU0 on/off

So far psci_cpu_(on|off) only worked for CPU1. Allow to control
CPU0 too. This allows to run the Linux PSCI checker successfully:
  [    2.213447] psci_checker: PSCI checker started using 2 CPUs
  [    2.219107] psci_checker: Starting hotplug tests
  [    2.223859] psci_checker: Trying to turn off and on again all CPUs
  [    2.267191] IRQ21 no longer affine to CPU0
  [    2.293266] Retrying again to check for CPU kill
  [    2.302269] CPU0 killed.
  [    2.311648] psci_checker: Trying to turn off and on again group 0 (CPUs 0-1)
  [    2.354354] IRQ21 no longer affine to CPU0
  [    2.383222] Retrying again to check for CPU kill
  [    2.392148] CPU0 killed.
  [    2.398063] psci_checker: Hotplug tests passed OK
  [    2.402910] psci_checker: Starting suspend tests (10 cycles per state)
  [    2.410019] psci_checker: cpuidle not available on CPU 0, ignoring
  [    2.416452] psci_checker: cpuidle not available on CPU 1, ignoring
  [    2.422757] psci_checker: Could not start suspend tests on any CPU
  [    2.429370] psci_checker: PSCI checker completed

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>