ynl-gen-cpp: add support for generating devlink ovs_datapath ovs_vport

Summary:
Enable code gen netlink for devlink ovs_datapath ovs_vport
devlink need support limits using definitions. backport logic from net-next

net-next:
1. tools: ynl-gen: use names of constants in generated limits(commit bcbbfaa2612d7d0e9c3eafade5f6e93c3672f34f)
2.tools: ynl-gen: support limits using definitions(commit fa796178e5eb0078a9a6c36f60fd6494cfc3f81d)

Test Plan:
make sure code could be generated and compiled

Signed-off-by: Sheldon Wu <xzw2048@gmail.com>

Author: sheldnwu

generated/Makefile

@@ -17,7 +17,7 @@ GENS_PATHS=$(shell grep -nrI --files-without-match \
 		'protocol: netlink' ../Documentation/netlink/specs/)
 GENS_PATHS+=$(wildcard ../Documentation/netlink/specs/rt-*.yaml)
 GENS_ALL=$(patsubst ../Documentation/netlink/specs/%.yaml,%,${GENS_PATHS})
-GENS=$(filter-out devlink ovs_datapath ovs_flow ovs_vport nlctrl,${GENS_ALL})
+GENS=$(filter-out ovs_flow nlctrl,${GENS_ALL})
 SRCS=$(patsubst %,%-user.cpp,${GENS})
 HDRS=$(patsubst %,%-user.hpp,${GENS})
 OBJS=$(patsubst %,%-user.cpp.o,${GENS})