General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/cpu/mpc512x/speed.c b/cpu/mpc512x/speed.c
index 5992111..1f90862 100644
--- a/cpu/mpc512x/speed.c
+++ b/cpu/mpc512x/speed.c
@@ -138,7 +138,7 @@
 
 U_BOOT_CMD(clocks, 1, 0, do_clocks,
 	"print clock configuration",
-	"    clocks\n"
+	"    clocks"
 );
 
 int prt_mpc512x_clks (void)