clk: mvebu: Use common of_clk_init() function

The use common of_clk_init() function simplifies the clock initialization
and adds handling of the DT "fixed-clock".

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed $SUBJECT to reflect correct file path]