tools: ynl: add support for encoding multi-attr

Multi-attr elements could not be encoded because of missing logic in the
ynl code. Enable encoding of these attributes by checking if the
attribute is a multi-attr and if the value to be processed is a list.

This has been tested both with the taprio and ets qdisc which contain
this kind of attributes.

Signed-off-by: Alessandro Marcolini <alessandromarcolini99@gmail.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/c5bc9f5797168dbf7a4379c42f38d5de8ac7f38a.1706962013.git.alessandromarcolini99@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>