doc: mkimage: Regularize option documentation

Square brackets are commonly used to denote optional parts of a command.
However, all option arguments are mandatory. Remove these brackets. This
also removes some unnecessary quotation marks, and uses hyphens to connect
words in option arguments. This is intended to just clean up the
formatting, leaving content corrections to later patches.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>