clk: Document of_parse_clkspec() some more

The return value of of_parse_clkspec() is peculiar. If the function is
called with a NULL argument for 'name' it will return -ENOENT, but if
it's called with a non-NULL argument for 'name' it will return -EINVAL.
This peculiarity is documented by commit 5c56dfe63b6e ("clk: Add comment
about __of_clk_get_by_name() error values").

Let's further document this function so that it's clear what the return
value is and how to use the arguments to parse clk specifiers.

Cc: Phil Edworthy <[email protected]>
Signed-off-by: Stephen Boyd <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Reviewed-by: Phil Edworthy <[email protected]>

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