cmd: pwm: Remove additional pwm description

The first name is taken from command name that's why shouldn't be listed in
help. And commands shouldn't be listed with <> which means value but value
itself is command name.
Also add description for commands to make it clear what it does.

Before
pwm pwm <invert> <pwm_dev_num> <channel> <polarity>
pwm <config> <pwm_dev_num> <channel> <period_ns> <duty_ns>
...

After:
pwm invert <pwm_dev_num> <channel> <polarity> - invert polarity
pwm config <pwm_dev_num> <channel> <period_ns> <duty_ns> - config PWM
pwm enable <pwm_dev_num> <channel> - enable PWM output
pwm disable <pwm_dev_num> <channel> - disable PWM output

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>