Input: imx_keypad - use clk_prepare_enable/clk_disable_unprepare()

Adapt clock handling to the new i.mx clock framework and fix the following
warning:

input: imx-keypad as /devices/platform/imx-keypad/input/input0
------------[ cut here ]------------
WARNING: at drivers/clk/clk.c:511 __clk_enable+0x98/0xa8()
Modules linked in:
[<c001a680>] (unwind_backtrace+0x0/0xf4) from [<c002452c>] (warn_slowpath_commo)
[<c002452c>] (warn_slowpath_common+0x48/0x60) from [<c0024560>] (warn_slowpath_)
[<c0024560>] (warn_slowpath_null+0x1c/0x24) from [<c02c4ec4>] (__clk_enable+0x9)
[<c02c4ec4>] (__clk_enable+0x98/0xa8) from [<c02c4ef8>] (clk_enable+0x24/0x5c)
[<c02c4ef8>] (clk_enable+0x24/0x5c) from [<c027ac6c>] (imx_keypad_open+0x28/0xc)
[<c027ac6c>] (imx_keypad_open+0x28/0xc8) from [<c0274b14>] (input_open_device+0)
[<c0274b14>] (input_open_device+0x78/0xa8) from [<c01ec884>] (kbd_connect+0x60/)
[<c01ec884>] (kbd_connect+0x60/0x80) from [<c0273b94>] (input_attach_handler+0x)
[<c0273b94>] (input_attach_handler+0x220/0x258) from [<c02755d4>] (input_regist)
[<c02755d4>] (input_register_device+0x31c/0x390) from [<c038da1c>] (imx_keypad_)
[<c038da1c>] (imx_keypad_probe+0x2e4/0x3b8) from [<c020326c>] (platform_drv_pro)
[<c020326c>] (platform_drv_probe+0x18/0x1c) from [<c0201f64>] (driver_probe_dev)
[<c0201f64>] (driver_probe_device+0x84/0x210) from [<c020217c>] (__driver_attac)
[<c020217c>] (__driver_attach+0x8c/0x90) from [<c02008f8>] (bus_for_each_dev+0x)
[<c02008f8>] (bus_for_each_dev+0x68/0x90) from [<c0201064>] (bus_add_driver+0xa)
[<c0201064>] (bus_add_driver+0xa4/0x23c) from [<c020275c>] (driver_register+0x7)
[<c020275c>] (driver_register+0x78/0x12c) from [<c00087c0>] (do_one_initcall+0x)
[<c00087c0>] (do_one_initcall+0x34/0x188) from [<c04b9310>] (kernel_init+0xe4/0)
[<c04b9310>] (kernel_init+0xe4/0x1a8) from [<c0015bd8>] (kernel_thread_exit+0x0)
---[ end trace 1d550e891d03d7ce ]---

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>