clk: sunxi: Remove use of variable length array

Using an array allocated on the stack may lead to stack overflows
and other problems so let's move the allocation to the heap
instead. This silences the following checker warning as well.

drivers/clk/sunxi/clk-sun8i-mbus.c:36:29: warning: Variable length array is used

Cc: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>