arm: mxc: utilise usecount field in clock operations

This patch fixes the clock refcounting when reparenting is used.

Boot-tested on imx51 babbage board.

Sascha pointed out a good explanation of refcounting here:
http://www.spinics.net/lists/arm-kernel/msg85879.html



Signed-off-by: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>