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: NMichal Simek <michal.simek@xilinx.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

cmd/pwm.c

@@ -108,7 +108,8 @@ static int do_pwm(struct cmd_tbl *cmdtp, int flag, int argc,
 
 U_BOOT_CMD(pwm, 6, 0, do_pwm,
 	   "control pwm channels",
-	   "pwm <invert> <pwm_dev_num> <channel> <polarity>\n"
-	   "pwm <config> <pwm_dev_num> <channel> <period_ns> <duty_ns>\n"
-	   "pwm <enable/disable> <pwm_dev_num> <channel>\n"
+	   "invert <pwm_dev_num> <channel> <polarity> - invert polarity\n"
+	   "pwm config <pwm_dev_num> <channel> <period_ns> <duty_ns> - config PWM\n"
+	   "pwm enable <pwm_dev_num> <channel> - enable PWM output\n"
+	   "pwm disable <pwm_dev_num> <channel> - eisable PWM output\n"
 	   "Note: All input values are in decimal");