clk: sunxi: sun6i-apb0-gates: use bitmaps for valid gate indices

sun6i-apb0-gates uses the "clock-indices" DT property to indicate
valid gate bits or holes in between. However, the rest of sunxi
clock drivers use bitmaps for this purpose.

This patch modifies sun6i-apb0-gates to use bitmaps as well, to be
consistent with the sunxi platform. Also add the missing call to
clk_register_clkdev, so system clock lookups will work.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>