ARM: mvebu: move DT boards to SoC-centric clock init

SoC centric clock init for Armada 370/XP can be used by calling of_clk_init.
Use it and get rid of mvebu_clocks_init.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>