Add uint24.yang

Add uint24.yang

odl-bgp-evpn defines an explicit uint24 type, which is useful as
low as network-concepts. Separate this type into its own module
and reuse it everywhere we are dealing with a 24 bit unsigned types.

This flushes out unneeded tests in LlGracefulCapabilityHandlerTest,
where we are asserting that DTOs are validating the value -- which
is a given and tested in DTO codegen tests.

Change-Id: I423d1af64b0514aa2b0e2f0f0fca728c062ef89a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>