Documentation: clk: enable lock is not held for clk_is_enabled API

The core does not need to hold enable lock for clk_is_enabled API.
Update the doc to reflect it.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
[sboyd: Clarified the last sentence a little more and fixed a spelling
error]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>