clk: Add write operation for clk_parent debugfs node

Useful for testing mux clocks. One can write the index of the parent to
be set into clk_parent node, starting from 0. Example

    # cd /sys/kernel/debug/clk/mout_peri_bus
    # cat clk_possible_parents
      dout_shared0_div4 dout_shared1_div4
    # cat clk_parent
      dout_shared0_div4
    # echo 1 > clk_parent
    # cat clk_parent
      dout_shared1_div4

CLOCK_ALLOW_WRITE_DEBUGFS has to be defined in drivers/clk/clk.c in
order to use this feature.

Signed-off-by: Sam Protsenko <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Reviewed-by: Geert Uytterhoeven <[email protected]>
Reviewed-by: Fabio Estevam <[email protected]>
Acked-by: Michael Turquette <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[[email protected]: Collapse ifdefs]
Signed-off-by: Stephen Boyd <[email protected]>

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