mlxsw: reg: Add a function to fill IPv6 unicast FDB entries

Add a function to fill IPv6 unicast FDB entries. Use the common function
for common fields.

Unlike IPv4 entries, the underlay IP address is not filled in the
register payload, but instead a pointer to KVDL is used.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>