common: command: Rework the 'cmd is repeatable' logic

The repeatable property is currently attached to the main command and
sub-commands have no way to change the repeatable value (the
->repeatable field in sub-command entries is ignored).

Replace the ->repeatable field by an extended ->cmd() hook (called
->cmd_rep()) which takes a new int pointer to store the repeatable cap
of the command being executed.

With this trick, we can let sub-commands decide whether they are
repeatable or not.

We also patch mmc and dtimg who are testing the ->repeatable field
directly (they now use cmd_is_repeatable() instead), and fix the help
entry manually since it doesn't use the U_BOOT_CMD() macro.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>