clk: sunxi: Add a simple gates driver

The gates were handled with a common piece of framework that was
registering all gates array, that was not using the CLK_OF_DECLARE logic,
and was not using clock-indices but some private masks that were pretty
much equivalent.

Move this code in a new driver that handles all the gates array and solves
both these issues.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
[sboyd@codeaurora.org: Include clk.h for consumer API usage]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>