tools: ynl: Add missing types to encode/decode

While testing the tool I noticed we miss the u16 type on payload create.
On the code inspection it turned out we miss also u64 - add them.

We also miss the decoding of u16 despite the fact `NlAttr` class
supports it - add it.

Signed-off-by: Michal Michalik <michal.michalik@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>