ynl: allow to encode u8 attr

Playing with dpll netlink, I came across following issue:
$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml --do pin-set --json '{"id": 0, "pin-idx": 1, "pin-state": 1}'
Traceback (most recent call last):
  File "tools/net/ynl/cli.py", line 52, in <module>
    main()
  File "tools/net/ynl/cli.py", line 40, in main
    reply = ynl.do(args.do, attrs)
  File "tools/net/ynl/lib/ynl.py", line 520, in do
    return self._op(method, vals)
  File "tools/net/ynl/lib/ynl.py", line 476, in _op
    msg += self._add_attr(op.attr_set.name, name, value)
  File "tools/net/ynl/lib/ynl.py", line 344, in _add_attr
    raise Exception(f'Unknown type at {space} {name} {value} {attr["type"]}')
Exception: Unknown type at dpll pin-state 1 u8

I'm not that familiar with ynl code, but from a quick peek, I suspect
that couple other types are missing for both encoding and decoding.
Ignoring those here as I'm scratching my local itch only.

Fix the issue by adding u8 attr packing.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230322154242.1739136-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Jiri Pirko

tools/net/ynl/lib/ynl.py

@@ -334,6 +334,8 @@ def _add_attr(self, space, name, value):
                 attr_payload += self._add_attr(attr['nested-attributes'], subname, subvalue)
         elif attr["type"] == 'flag':
             attr_payload = b''
+        elif attr["type"] == 'u8':
+            attr_payload = struct.pack("B", int(value))
         elif attr["type"] == 'u32':
             attr_payload = struct.pack("I", int(value))
         elif attr["type"] == 'string':