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 <semen.protsenko@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Michael Turquette <mturquette@baylibre.com>
Link: https://lore.kernel.org/r/20211013172042.10884-1-semen.protsenko@linaro.org
[sboyd@kernel.org: Collapse ifdefs]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

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