ARM: orion5x: switch to use the clock driver for DT platforms

This commit moves the Orion5x platforms using the Device Tree to use
the recently introduced clock driver for Orion5x. To achieve that, it:

 * Adds the necessary DT description of the clock.

 * Selects ORION_CLK to enable the compilation of the clock driver.

 * Call of_clk_init() instead of the Orion5x-specific clock
   initialization function.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1398202002-28530-21-git-send-email-thomas.petazzoni@free-electrons.com
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>