clk: sunxi: Rename the structure to prepare the addition of sun6i

Rename all the generic-named structure to sun4i to avoid confusion when
we will introduce the sun6i (A31) clocks.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Emilio López <emilio@elopez.com.ar>