tools: ynl-gen: inherit policy in multi-attr

Instead of reimplementing policies in MutliAttr for every
underlying type forward the calls to the base type.
This will be needed for DPLL which uses a multi-attr nest,
and currently gets an invalid NLA_NEST policy generated.

Reviewed-by: Jacob Keller <Jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>