Default nums of RX/TX descriptors changed to 512 for 2M page on DVN
Default nums of RX/TX descriptors changed to 512 for 2M page on DVN

As DVN has fewer DTLB entries supported for 2M page, default numbers of
RX/TX descriptors are changed to 512 if nums of RX/TX descriptors are not
specified by VPP users.

Change-Id: I076493b802b15d12750a5b49d1554da4d19ad460
Signed-off-by: Zhiyong Yang <fbdb847fac8e5e50b588eb93d9ff9fd811b7568c@intel.com>

chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp