cmd: mtd: add benchmark option to the help

cmd: mtd: add benchmark option to the help

The patch adds benchmark option to the help of mtd command. For the
'mtd write' case the help line exceed 80 characters. Ignore this issue
as modern terminals are capable to handle more characters.

The patch also formats other command to make sure all device names
starts on the same collumn.

Fixes: d246e70cf81d0 ("cmd: mtd: Enable speed benchmarking")
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>