clk: rockchip: remove deprecated gate-clk code and dt-binding

Initially we tried modeling clocks via the devicetree before switching
to clocks declared in the clock drivers and only exporting specific
ids to the devicetree.

As the old code was in the kernel for 1-2 releases when the new mode
of operation was added we kept it for backwards compatibility.

That deprecation notice is in the binding since july 2014, so nearly
4 years now and I think it's time to drop the old cruft.

Especially as at the time using the mainline kernel on Rockchip devices
was not really possible, except for experiments on the really old socs of
the rk3066 + rk3188 line, so there shouldn't be any devicetrees still
around that rely on that code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>