linux-imx-rt: Use 'performance' as default cpufreq governor

linux-imx-rt: Use 'performance' as default cpufreq governor

Since CPU frequency scaling is a bad idea for real-time systems anyway,
use 'performance' and disable the remaining cpufreq governors.

Avoids the following issue:

  BUG: scheduling while atomic: swapper/0/0/0x00000002
  Modules linked in:
  Preemption disabled at:[<  (null)>]   (null)

  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.10.17-rt12-monkey+g232293e #1
  [<80013c7c>] (unwind_backtrace+0x0/0xec) from [<80011644>] (show_stack+0x10/0x14)
  [<80011644>] (show_stack+0x10/0x14) from [<8004e160>] (__schedule_bug+0x60/0xa8)
  [<8004e160>] (__schedule_bug+0x60/0xa8) from [<80643458>] (__schedule+0x408/0x4a8)
  [<80643458>] (__schedule+0x408/0x4a8) from [<8064352c>] (schedule+0x34/0xa4)
  [<8064352c>] (schedule+0x34/0xa4) from [<80644104>] (rt_spin_lock_slowlock+0xc0/0x298)
  [<80644104>] (rt_spin_lock_slowlock+0xc0/0x298) from [<80033780>] (lock_timer_base.isra.29+0x2c/0x4c)
  [<80033780>] (lock_timer_base.isra.29+0x2c/0x4c) from [<80033944>] (mod_timer+0x60/0x1c4)
  [<80033944>] (mod_timer+0x60/0x1c4) from [<8044bcc8>] (cpufreq_interactive_idle_notifier+0x148/0x180)
  [<8044bcc8>] (cpufreq_interactive_idle_notifier+0x148/0x180) from [<8004ad40>] (notifier_call_chain+0x44/0x84)
  [<8004ad40>] (notifier_call_chain+0x44/0x84) from [<8004afd4>] (atomic_notifier_call_chain+0x34/0x48)
  [<8004afd4>] (atomic_notifier_call_chain+0x34/0x48) from [<8005d9a0>] (cpu_startup_entry+0xfc/0x164)
  [<8005d9a0>] (cpu_startup_entry+0xfc/0x164) from [<80c1cab4>] (start_kernel+0x324/0x330)
  [<80c1cab4>] (start_kernel+0x324/0x330) from [<10008074>] (0x10008074)

See:
  https://lkml.org/lkml/2014/6/10/457
  https://lists.yoctoproject.org/pipermail/meta-freescale/2014-June/008957.html

Signed-off-by: Jacob Kroon <jacob.kroon@mikrodidakt.se>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>