[yang] Support new device type

[ganglyu]

Description

minigraph has introduced new device type: BmcMgmtToRRouter
https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-config-engine/minigraph.py#L40
Yang models does not support this pattern:
https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-yang-models/yang-models/sonic-device_metadata.yang#L88-L93

Steps to reproduce the issue:

1. Run unit test for sonic-config-engine
2. Check output device type

Describe the results you received:

BmcMgmtToRRouter can't pass yang validation.

Describe the results you expected:

BmcMgmtToRRouter should pass yang validation.

Output of show version:

(paste your output here)

Output of show techsupport:

(paste your output here or download and attach the file here )

Additional information you deem important (e.g. issue happens only occasionally):