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 <phil.edworthy@renesas.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190826212042.48642-1-sboyd@kernel.org
Reviewed-by: Phil Edworthy <phil.edworthy@renesas.com>

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