common: command: Expose a generic helper to auto-complete sub commands

Some commands have a table of sub-commands. With minor adjustments,
complete_cmdv() is able to provide auto-completion for sub-commands
(it's just about passing the table of commands instead of taking the
global one).
We rename this function into complete_subcmd() and implement
complete_cmdv() as a wrapper around complete_subcmdv().

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