docs: clk: add documentation to log which clocks have been disabled

docs: clk: add documentation to log which clocks have been disabled

The existing clk documentation has a section that talks about the
clk_ignore_unused kernel parameter. Add additional documentation that
describes how to log which clocks the kernel disables on bootup. This
will log messages like the following to the console on bootup:

    [    1.268115] clk: Disabling unused clocks
    [    1.272167] clk_disable: gcc_usb_clkref_en
    [    1.276389] clk_disable: gcc_usb30_sec_sleep_clk
    [    1.281131] clk_disable: gcc_usb30_prim_sleep_clk
    ...

Signed-off-by: Brian Masney <bmasney@redhat.com>
Link: https://lore.kernel.org/r/20230411192153.289688-1-bmasney@redhat.com
[jc: turned parameters into a literal block]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>