clk: gpio: Use DT way of specifying parents

Nobody has used the gpio clk registration functions nor the gpio clk_ops
exposed by the basic gpio clk type. Let's remove all those APIs and move
the gpio clk support into the C file. Since nothing is using the
exported APIs, simplify the driver to be a platform driver that uses
clk_parent_data to pick 0th or 1st cell of the node's clocks property.

Cc: Simon Horman <horms@verge.net.au>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190830150923.259497-2-sboyd@kernel.org