clk: sunxi-ng: r40: rewrite init code to a platform driver

As we need to register a regmap on the R40 CCU, there needs to be a
device structure bound to the CCU device node.

Rewrite the R40 CCU driver initial code to make it a proper platform
driver, thus we will have a platform device bound to it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>