rockchip: clk: Make rkclk_get_clk() SoC-specific

rockchip: clk: Make rkclk_get_clk() SoC-specific

The current method assumes that clocks are numbered from 0 and we can
determine a clock by its number. It is safer to use an ID in the clock's
platform data to avoid the situation where another clock is bound before
the one we expect.

Move the existing code into rk3036 since it still works there. Add a new
implementation for rk3288.

Signed-off-by: Simon Glass <sjg@chromium.org>