[YANG] leaf type missing SonicDPU in sonic-device_metadata.yang

[wen587]

Description

Steps to reproduce the issue:

1. Checkout json file for dpu and yang validation failed https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-config-engine/tests/sample_output/t1-smartswitch-dpu.json
   Introduced PR: [smart-switch] Extend config generator for t1-smartswitch topology. #19056

libyang[0]: Value "SonicDpu" does not satisfy the constraint "ToRRouter|LeafRouter|SpineChassisFrontendRouter|ChassisBackendRouter|ASIC|MgmtToRRouter|SpineRouter|BackEndToRRouter|BackEndLeafRouter|EPMS|MgmtTsToR|BmcMgmtToRRouter|not-provisioned" (range, length, or pattern). (path: /sonic-device_metadata:sonic-device_metadata/DEVICE_METADATA/localhost/type)
sonic_yang(3):Data Loading Failed:Value "SonicDpu" does not satisfy the constraint "ToRRouter|LeafRouter|SpineChassisFrontendRouter|ChassisBackendRouter|ASIC|MgmtToRRouter|SpineRouter|BackEndToRRouter|BackEndLeafRouter|EPMS|MgmtTsToR|BmcMgmtToRRouter|not-provisioned" (range, length, or pattern).

Describe the results you received:

Fail YANG

Describe the results you expected:

Pass YANG

Output of show version:

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Output of show techsupport:

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