#### Why I did it
Add new SKU for SN2700 Mellanox system that supports the following port configuration:
8 X 100G
40 X 50G
8 X 10G
#### How I did it
Add new Folder - "Mellanox-SN2700-D40C8S8" under /sonic-buildimage/device/mellanox/x86_64-mlnx_msn2700-r0/
that contains the relevant files supporting this SKU
the buffers are based on SKU: D48C8 . Later on it will be configured specific for this SKU
#### How to verify it
Bring up the image, run "show interface status" and make sure that all ports are up and reflect the following requirement:
Port 1/3 will be used as 4x10G
Port 2/4 - Not exist (blocked since 1 and 3 split to 4)
Port 7/8/9/10/23/24/25/26 will used as 100G
All other ports will be used as 2x50G
#### Which release branch to backport (provide reason below if selected)
- [ ] 201811
- [ ] 201911
- [ ] 202006
- [X] 202012
#### Description for the changelog
Support new SKU under the name of SN2700-D40C8S8
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