ARM: imx: avoid calling clk APIs in idle thread which may cause schedule

As clk_pllv3_wait_lock will call usleep_range, and the clk APIs
mutex lock may be held when CPU entering idle, so calling clk
APIs must be avoided in cpu idle thread, this is to avoid reschedule
warning in cpu idle, just access register directly to achieve that.

bad: scheduling from the idle thread!
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.14.0-rc1+ #657
Backtrace:
[<80012188>] (dump_backtrace) from [<8001246c>] (show_stack+0x18/0x1c)
 r6:808c0038 r5:00000000 r4:808e5a1c r3:00000000
[<80012454>] (show_stack) from [<8064b2ec>] (dump_stack+0x84/0x9c)
[<8064b268>] (dump_stack) from [<80055ee0>] (dequeue_task_idle+0x20/0x30)
 r5:808bef40 r4:bf7dff40
[<80055ec0>] (dequeue_task_idle) from [<8004f028>] (dequeue_task+0x30/0x50)
 r4:bf7dff40 r3:80055ec0
[<8004eff8>] (dequeue_task) from [<800503c0>] (deactivate_task+0x30/0x34)
 r4:bf7dff40
[<80050390>] (deactivate_task) from [<8064d8e4>] (__schedule+0x2c8/0x5c0)
[<8064d61c>] (__schedule) from [<8064dc14>] (schedule+0x38/0x88)
 r10:80912964 r9:808c1e50 r8:808c0038 r7:808cbf30 r6:80e128ec r5:60000093
 r4:80912968
[<8064dbdc>] (schedule) from [<8064dfec>] (schedule_preempt_disabled+0x10/0x14)
[<8064dfdc>] (schedule_preempt_disabled) from [<8064ebc0>] (mutex_lock_nested+0x1c0/0x3c0)
[<8064ea00>] (mutex_lock_nested) from [<804ae71c>] (clk_prepare_lock+0x44/0xe4)
 r10:806554cc r9:bf7df1bc r8:808cf4f8 r7:808cf544 r6:bf7df1b8 r5:808c0010
 r4:80e69750
[<804ae6d8>] (clk_prepare_lock) from [<804af214>] (clk_get_rate+0x14/0x64)
 r6:bf7df1b8 r5:00000002 r4:bf017000 r3:80922ad0
[<804af200>] (clk_get_rate) from [<80025d30>] (imx6sl_set_wait_clk+0x18/0x20)
 r5:00000002 r4:00000001
[<80025d18>] (imx6sl_set_wait_clk) from [<80023454>] (imx6sl_enter_wait+0x20/0x48)
[<80023434>] (imx6sl_enter_wait) from [<80477c24>] (cpuidle_enter_state+0x44/0xfc)
 r4:3c386e48 r3:80023434
[<80477be0>] (cpuidle_enter_state) from [<80477dd8>] (cpuidle_idle_call+0xfc/0x160)
 r8:808cf4f8 r7:00000001 r6:80e69534 r5:00000000 r4:bf7df1b8
[<80477cdc>] (cpuidle_idle_call) from [<8000f61c>] (arch_cpu_idle+0x10/0x50)
 r9:808c0000 r8:00000000 r7:80921a89 r6:808c8938 r5:808c899c r4:808c0000
[<8000f60c>] (arch_cpu_idle) from [<8006fa94>] (cpu_startup_entry+0x108/0x160)
[<8006f98c>] (cpu_startup_entry) from [<806452ac>] (rest_init+0xb4/0xdc)
 r7:808afae0
[<806451f8>] (rest_init) from [<8086fb58>] (start_kernel+0x328/0x38c)
 r6:ffffffff r5:808c8880 r4:808c8a30
[<8086f830>] (start_kernel) from [<80008074>] (0x80008074)

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>