samples: net: dsa: rework for flexibility and extensibility

Current DSA sample is initially for ip_k66f board with a switch PHY,
while it is expected as a common DSA sample for all DSA devices.

This patch does not change any function. It only reworks the sample
with new added CONFIG_NET_SAMPLE_DSA_LLDP for ip_k66f with 3 LAN
ports.
Ideally this should be common for all DSA devices, but for now, ip_k66f
is the only platform supporting it.
As there will be more DSA functions supported, better to allow each
function enablement via option to suit hardware or device driver support
status.

Also improved dsa_ll_addr_switch_cb return value with enum value instead
of plain value.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>