tools/net/ynl: support decoding indexed arrays as enums

When decoding an indexed-array with a scalar subtype, it is currently
only possible to add a display-hint. Add support for decoding each value
as an enum.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250211120127.84858-3-donald.hunter@gmail.com


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