tools: ynl-gen: support local attrs in _multi_parse

The _multi_parse() helper calls the _attr_get() method of each attr,
but it only respects what code the helper wants to emit, not what
local variables it needs. Local variables will soon be needed,
support them.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250520161916.413298-9-kuba@kernel.org


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