clk: clk-gpio: update documentation for gpio-gate clock

clk: clk-gpio: update documentation for gpio-gate clock

The main documentation block seems to be from a time before the driver
handled sleeping and non-sleeping gpios and with that change it seems
updating the doc was overlooked. So do that now.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20240906082511.2963890-3-heiko@sntech.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>