clk: sunxi: declare OF clock provider

Common clock framework allows to register clock providers to get called
on of_clk_init() by using CLK_OF_DECLARE. This converts sunxi clock
providers to make use of it and get rid of the mach specific clk init
call. As sunxi has a bunch of independent clk provider nodes, we hook
current clock init to board compatible to make it called once.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>