scripts: west_commands: fix type of path options

Using `west patch` with the `--patch-base`, `--patch-yaml` or
`--west-workspace` option results in an unhandled AttributeError in
filter_args() because the function expects these args to be of type
path. Adding the type to the argument definition forces argparse to
create variables of type path.

Signed-off-by: Gerhard Jörges <joerges@metratec.com>