cmd: led: fix help text for blink subcommand

cmd: led: fix help text for blink subcommand

The blink subcommand actually requires an additional parameter
(blink-freq) but not the others.

In order to simplify the help text, split the blink subcommand help text
from the off|on|toggle subcommands.

Then, fix the help text so that it is clear that the frequency is
required.

While at it, specify the duty cycle.

Fixes: ffe2052d6e8a ("dm: led: Add a new 'led' command")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>