clk: sunxi: Use CLK_IS_CRITICAL flag for critical clks

We'd like to privatize __clk_get(), but the sunxi clk driver is
calling this function to keep a reference held on the clk and
call clk_prepare_enable() on it. We support this design in the
clk core now with the CLK_IS_CRITICAL flag, so let's just use
that instead.

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