mx2_camera: Fix regression caused by clock conversion

Since mx27 transitioned to the commmon clock framework in 3.5, the correct way
to acquire the csi clock is to get csi_ahb and csi_per clocks separately.

By not doing so the camera sensor does not probe correctly:

soc-camera-pdrv soc-camera-pdrv.0: Probing soc-camera-pdrv.0
mx2-camera mx2-camera.0: Camera driver attached to camera 0
ov2640 0-0030: Product ID error fb:fb
mx2-camera mx2-camera.0: Camera driver detached from camera 0
mx2-camera mx2-camera.0: MX2 Camera (CSI) driver probed, clock frequency: 66500000

Adapt the mx2_camera driver to the new clock framework and make it functional
again.

Tested-by: Gaëtan Carlier <gcembed@gmail.com>
Tested-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>