tools: ynl: Use dict of predefined Structs to decode scalar types

Use a dict of predefined Struct() objects to decode scalar types in native,
big or little endian format. This removes the repetitive code for the
scalar variants and ensures all the signed variants are supported.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>