ARM: mx5: Replace clk_register_clkdev with clock DT lookup

Similarly as it was done for mx6q, use a DT lookup in order to make maintainance
task for the clock devices easier.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>