powerpc/512x: clk: minor comment updates

adjust (expand on or move) a few comments,
add markers for easier navigation around helpers

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>