clk: sunxi: Fix gate indexing for sun6i-a31-apb0-gates

sun6i-a31-apb0-gates supports using clock-indices for holes between
individual gates. However, the driver passes the number of gates
registered in clk_data->clk_num, which of_clk_src_onecell_get uses
to recognize the range of valid indices a consumer can use.

This patch makes the driver pass the maximum gate index + 1, so
of_clk_src_onecell_get does not complain about indices greater
than gates registered.

This was tested on the A23 SoC, which has a similar APB0 clock,
but has holes for gates to removed IP blocks.

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